Have git module use sys.platform to check for Windows

This changes the way code throughout the git module checks to see
if it is running on a native Windows system.

Some checks using os.name were already changed to use sys.platform
in dc95a76 and 4191f7d. (See dc95a76 on why the specific question
of whether the system is native Windows can be answered by either
checking os.name or sys.platform, even though in general they
differ in granularity and are not always suited to the same tasks.)

The reasons for this change are:

- Consistency: Due to dc95a76 and 4191f7d, some of these checks use
  sys.platform, so in the absence of a reason to do otherwise, it
  is best that they all do. Otherwise, it creates an appearance
  that the technical reasons behind the difference are stronger
  than they really are, or even that differnt things are being
  checked.

- Better static typing: mypy treats sys.platform as a constant
  (and also allows checking on platform on another via --platform).
  Likewise, typeshed conditions platform-dependent declarations on
  sys.platform checks, rather than os.name or other checks. Really
  this is the original reason for those earlier, more selective
  changes, but here the goal is more general, since this is not
  needed to address any specific preexisting mypy errors.

This is incomplete, for two reasons:

- I'm deliberately not including changes in the tests in this
  commit. Arguably it should be kept as os.name in the tests, on
  the grounds that the test suite is not currently statically typed
  anyway, plus having them differ more compellingly shows that the
  behavior is the same whether an os.name or sys.platform check is
  used. However, it would be confusing to keep them different, and
  also somewhat unnatural; one approach would probably end up
  leaking through. Furthermore, because some tests have to check
  for cygwin specifically, which cannot be done with os.name, it
  may be clearer to use sys.platform for all platform checking in
  the test suite. But to verify and demonstrate that the change
  really is safe, I'm waiting to make the change in the tests until
  after making them in the code under test.

- Some forms of checks against constants produce unreachable code
  errors from mypy (python/mypy#10773).
  This can be worked around, but I have not done that in this
  commit. Furthermore, the new mypy errors this produces--one on
  Windows, and one on non-Windows systems--could be fixed in a way
  that makes type annotations richer, by allowing the return type
  to be a literal on one platform or the other.

Author: EliahKagan

Diff for: git/config.py

@@ -246,7 +246,7 @@ def items_all(self) -> List[Tuple[str, List[_T]]]:
 def get_config_path(config_level: Lit_config_levels) -> str:
     # We do not support an absolute path of the gitconfig on Windows.
     # Use the global config instead.
-    if os.name == "nt" and config_level == "system":
+    if sys.platform == "win32" and config_level == "system":
         config_level = "global"
 
     if config_level == "system":

Diff for: git/index/base.py

@@ -13,6 +13,7 @@
 import os
 from stat import S_ISLNK
 import subprocess
+import sys
 import tempfile
 
 from git.compat import (
@@ -107,7 +108,7 @@ def _named_temporary_file_for_subprocess(directory: PathLike) -> Generator[str,
         A context manager object that creates the file and provides its name on entry,
         and deletes it on exit.
     """
-    if os.name == "nt":
+    if sys.platform == "win32":
         fd, name = tempfile.mkstemp(dir=directory)
         os.close(fd)
         try:

Diff for: git/index/fun.py

@@ -18,6 +18,7 @@
     S_IXUSR,
 )
 import subprocess
+import sys
 
 from git.cmd import handle_process_output, safer_popen
 from git.compat import defenc, force_bytes, force_text, safe_decode
@@ -99,7 +100,7 @@ def run_commit_hook(name: str, index: "IndexFile", *args: str) -> None:
     env["GIT_EDITOR"] = ":"
     cmd = [hp]
     try:
-        if os.name == "nt" and not _has_file_extension(hp):
+        if sys.platform == "win32" and not _has_file_extension(hp):
             # Windows only uses extensions to determine how to open files
             # (doesn't understand shebangs). Try using bash to run the hook.
             relative_hp = Path(hp).relative_to(index.repo.working_dir).as_posix()

Diff for: git/objects/submodule/base.py

@@ -7,6 +7,7 @@
 import os
 import os.path as osp
 import stat
+import sys
 import uuid
 
 import git
@@ -401,7 +402,7 @@ def _write_git_file_and_module_config(cls, working_tree_dir: PathLike, module_ab
         """
         git_file = osp.join(working_tree_dir, ".git")
         rela_path = osp.relpath(module_abspath, start=working_tree_dir)
-        if os.name == "nt" and osp.isfile(git_file):
+        if sys.platform == "win32" and osp.isfile(git_file):
             os.remove(git_file)
         with open(git_file, "wb") as fp:
             fp.write(("gitdir: %s" % rela_path).encode(defenc))

Diff for: git/repo/base.py

@@ -12,6 +12,7 @@
 from pathlib import Path
 import re
 import shlex
+import sys
 import warnings
 
 import gitdb
@@ -336,10 +337,10 @@ def close(self) -> None:
             # they are collected by the garbage collector, thus preventing deletion.
             # TODO: Find these references and ensure they are closed and deleted
             # synchronously rather than forcing a gc collection.
-            if os.name == "nt":
+            if sys.platform == "win32":
                 gc.collect()
             gitdb.util.mman.collect()
-            if os.name == "nt":
+            if sys.platform == "win32":
                 gc.collect()
 
     def __eq__(self, rhs: object) -> bool:
@@ -618,7 +619,7 @@ def _get_config_path(self, config_level: Lit_config_levels, git_dir: Optional[Pa
             git_dir = self.git_dir
         # We do not support an absolute path of the gitconfig on Windows.
         # Use the global config instead.
-        if os.name == "nt" and config_level == "system":
+        if sys.platform == "win32" and config_level == "system":
             config_level = "global"
 
         if config_level == "system":

Diff for: git/util.py

@@ -117,7 +117,7 @@ def _read_win_env_flag(name: str, default: bool) -> bool:
     :note:
         This only accesses the environment on Windows.
     """
-    if os.name != "nt":
+    if sys.platform != "win32":
         return False
 
     try:
@@ -223,7 +223,7 @@ def handler(function: Callable, path: PathLike, _excinfo: Any) -> None:
                 raise SkipTest(f"FIXME: fails with: PermissionError\n  {ex}") from ex
             raise
 
-    if os.name != "nt":
+    if sys.platform != "win32":
         shutil.rmtree(path)
     elif sys.version_info >= (3, 12):
         shutil.rmtree(path, onexc=handler)
@@ -235,7 +235,7 @@ def rmfile(path: PathLike) -> None:
     """Ensure file deleted also on *Windows* where read-only files need special
     treatment."""
     if osp.isfile(path):
-        if os.name == "nt":
+        if sys.platform == "win32":
             os.chmod(path, 0o777)
         os.remove(path)
 
@@ -276,7 +276,7 @@ def join_path(a: PathLike, *p: PathLike) -> PathLike:
     return path
 
 
-if os.name == "nt":
+if sys.platform == "win32":
 
     def to_native_path_windows(path: PathLike) -> PathLike:
         path = str(path)
@@ -328,7 +328,7 @@ def _get_exe_extensions() -> Sequence[str]:
     PATHEXT = os.environ.get("PATHEXT", None)
     if PATHEXT:
         return tuple(p.upper() for p in PATHEXT.split(os.pathsep))
-    elif os.name == "nt":
+    elif sys.platform == "win32":
         return (".BAT", "COM", ".EXE")
     else:
         return ()
@@ -354,7 +354,9 @@ def is_exec(fpath: str) -> bool:
         return (
             osp.isfile(fpath)
             and os.access(fpath, os.X_OK)
-            and (os.name != "nt" or not winprog_exts or any(fpath.upper().endswith(ext) for ext in winprog_exts))
+            and (
+                sys.platform != "win32" or not winprog_exts or any(fpath.upper().endswith(ext) for ext in winprog_exts)
+            )
         )
 
     progs = []
@@ -451,8 +453,8 @@ def is_cygwin_git(git_executable: PathLike) -> bool:
 
 
 def is_cygwin_git(git_executable: Union[None, PathLike]) -> bool:
-    if os.name == "nt":
-        # This is Windows-native Python, since Cygwin has os.name == "posix".
+    if sys.platform == "win32":
+        # This is Windows-native Python, since Cygwin has sys.platform == "cygwin".
         return False
 
     if git_executable is None: