Let subprocess write temp file on Windows

On Windows, a file created by NamedTemporaryFile cannot be reopened
while already open, under most circumstances. This applies to
attempts to open it both from the original process and from other
processes. This differs from the behavior on other operating
systems, and it works this way to help ensure the file can be
automatically deleted (since having a file open on Windows usually
prevents the file from being deleted, unlike on other OSes).

However, this can cause problems when NamedTemporaryFile is used to
produce a safe filename for opening with an external process, as
IndexFile.from_tree does. On Windows, the git subprocess can't open
and write to the file. This breaks IndexFile.from_tree on Windows,
causing gitpython-developers#1630 (because IndexFile.reset uses IndexFile.from_tree,
and the Repo.index property returns an IndexFile instance, so
Repo.index.reset fails), and causes a number of other breakages.

This commit is an initial fix for the problem, but likely not the
best fix. On Windows, it closes the temporary file created by
NamedTemporaryFile, so the file can be opened by name, including by
a git subprocess. It is less than ideal because it at least appears
to create a race condition: without furher changes not yet made
here, closing the file actually deletes it, which conveniently lets
the name be reused, but because the file does not exist, the
filename might end up being reused before it is recreated.

The following 8 tests go from failing to passing due to this change
on Windows (as listed in the summary at the end of pytest output):

- test/test_docs.py:210 Tutorials.test_references_and_objects
- test/test_fun.py:37 TestFun.test_aggressive_tree_merge
- test/test_index.py:781 TestIndex.test_compare_write_tree
- test/test_index.py:294 TestIndex.test_index_file_diffing
- test/test_index.py:182 TestIndex.test_index_file_from_tree
- test/test_index.py:232 TestIndex.test_index_merge_tree
- test/test_index.py:428 TestIndex.test_index_mutation
- test/test_refs.py:218 TestRefs.test_head_reset

Author: EliahKagan

Diff for: git/index/base.py

@@ -362,6 +362,8 @@ def from_tree(cls, repo: "Repo", *treeish: Treeish, **kwargs: Any) -> "IndexFile
         # works - /tmp/ dirs could be on another device.
         with ExitStack() as stack:
             tmp_index = stack.enter_context(tempfile.NamedTemporaryFile(dir=repo.git_dir))
+            if os.name == "nt":
+                tmp_index.close()
             arg_list.append("--index-output=%s" % tmp_index.name)
             arg_list.extend(treeish)