Fix new mypy error in _read_win_env_flag

As noted in 88557bc before this, that change added a couple of new
mypy errors about unreachable code, where it is intentionally
unreachable because it is for one platform and not another.

This fixes one of them, though a fix that incorporates more of what
can now be statically known about the return value based on the
platform may be preferable.

Author: EliahKagan

Diff for: git/util.py

@@ -107,19 +107,12 @@
 _logger = logging.getLogger(__name__)
 
 
-def _read_win_env_flag(name: str, default: bool) -> bool:
-    """Read a boolean flag from an environment variable on Windows.
+def _read_env_flag(name: str, default: bool) -> bool:
+    """Read a boolean flag from an environment variable.
 
     :return:
-        On Windows, the flag, or the `default` value if absent or ambiguous.
-        On all other operating systems, ``False``.
-
-    :note:
-        This only accesses the environment on Windows.
+        The flag, or the `default` value if absent or ambiguous.
     """
-    if sys.platform != "win32":
-        return False
-
     try:
         value = os.environ[name]
     except KeyError:
@@ -140,6 +133,19 @@ def _read_win_env_flag(name: str, default: bool) -> bool:
     return default
 
 
+def _read_win_env_flag(name: str, default: bool) -> bool:
+    """Read a boolean flag from an environment variable on Windows.
+
+    :return:
+        On Windows, the flag, or the `default` value if absent or ambiguous.
+        On all other operating systems, ``False``.
+
+    :note:
+        This only accesses the environment on Windows.
+    """
+    return sys.platform == "win32" and _read_env_flag(name, default)
+
+
 #: We need an easy way to see if Appveyor TCs start failing,
 #: so the errors marked with this var are considered "acknowledged" ones, awaiting remedy,
 #: till then, we wish to hide them.