This removes the xfail marks from 8 tests that fail due to gitpython-developers#1630,
to be fixed in the subsequent commits.

This reverts commit 6e477e3.

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 accessing with an external process, as
IndexFile.from_tree does. On Windows, git subprocess can't open and
write to the file. This breaks IndexFile.from_tree on Windows. This
is the cause of gitpython-developers#1630 -- IndexFile.reset uses IndexFile.from_tree,
and the Repo.index property returns an IndexFile instance, so
Repo.index.reset fails -- and some other breakages.

While passing delete=False to NamedTemporaryFile (and deleting
separately) often overcomes that, it won't fix things here, because
IndexFile.from_tree uses "git read-tree --index-output=<file>".
That needs more than the ability to open the file for write. It
needs to be able to create or overwrite the given path by renaming
an existing file to it. The description of "--index-output=<file>"
in git-read-tree(1) notes:

    The file must allow to be rename(2)ed into from a temporary
    file that is created next to the usual index file; typically
    this means it needs to be on the same filesystem as the index
    file itself, and you need write permission to the directories
    the index file and index output file are located in.

On Windows, more is required: there must be no currently open file
with that path, because on Windows, open files typically cannot be
replaced, just as, on Windows, they typically cannot be deleted
(nor renamed). This is to say that passing delete=False to
NamedTemporaryFile isn't enough to solve this because it merely
causes NamedTemporaryFile to behave like the lower-level mkstemp
function, leaving the responsibility of deletion to the caller but
still *opening* the file -- and while the file is open, the git
subprocess can't replace it. Switching to mkstemp, with no further
change, would likewise not solve this.

This commit is an initial fix for the problem, but 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, and also overwritten, include by a file move.

As currently implemented, this is not ideal, as it creates a race
condition, because closing the file actually deletes it. During the
time the file does not exist, the filename may end up being reused
by another call to NamedTemporaryFile, mkstemp, or similar facility
(in any process, written in any language) before the git subprocess
can recreate it. Passing delete=False would avoid this, but then
deletion would have to be handled separately, and at that point it
is not obvious that NamedTemporaryFile is the best facility to use.

This fix, though not yet adequate, confirms the impact on gitpython-developers#1630.
The following 8 tests go from failing to passing due to this change
on a local Windows development machine (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

On CI, one test still fails, but later and for an unrelated reason:

- test/test_index.py:428 TestIndex.test_index_mutation

That `open(fake_symlink_path, "rt")` call raises FileNotFoundError.

The test assumes symlinks are never created on Windows. This often
turns out to be correct, because core.symlinks defaults to false on
Windows. (On some Windows systems, creating them is a privileged
operation; this can be reconfigured, and unprivileged creation of
symlinks is often enabled on systems used for development.)

However, on a Windows system with core.symlinks set to true, git
will create symlinks if it can, when checking out entries committed
to a repository as symlinks. GitHub Actions runners for Windows do
this ever since actions/runner-images#1186
(the file is now at images/windows/scripts/build/Install-Git.ps1;
the `/o:EnableSymlinks=Enabled` option continues to be passed,
causing Git for Windows to be installed with core.symlinks set to
true in the system scope).

For now, this adds an xfail marking to test_index_mutation, for the
FileNotFoundError raised when a symlink, which is expected not to
be a symlink, is passed to `open`, causing an attempt to open its
nonexistent target. (The check itself might bear refinement: as
written, it reads the core.symlinks variable from any scope,
including the local scope, which at the time of the check will
usually be the cloned GitPython directory, where pytest is run.)

While adding an import, this commit also improves the grouping and
sorting of existing ones.

This lets the Windows subprocess open or rename onto the temporary
file using a more robust approach than in b12fd4a, avoiding the
race condition described there, where the filename could be
inadvertently reused between deletion and recreation of the file.

This creates a context manager helper for the temporary index file
used in IndexFile.from_tree, whose implementation differs by
operating system:

- Delegating straightforwardly to NamedTempoaryFile on POSIX
  systems where an open file can replaced by having another file
  renamed to it (just as it can be deleted).

- Employing custom logic on Windows, using mkstemp, closing the
  temporary file without immediately deleting it (so it won't be
  reused by any process seeking to create a temporary file), and
  then deleting it on context manager exit.

IndexFile.from_tree now calls this helper instead of
NamedTemporaryFile. For convenience, the helper provides the path,
i.e. the "name", when entered, so tmp_index is now just that path.

(At least for now, this helper is implemented as a nonpublic
function in the git.index.base module, rather than in the
git.index.util module. If it were public in git.index.util like the
other facilities there, then some later changes to it, or its later
removal, would be a breaking change to GitPython. If it were
nonpublic in git.index.util, then this would not be a concern, but
it would be unintuitive for it to be accessed from code in the
git.index.base module. In the future, one way to address this might
be to have one or more nonpublic _util modules with public members.
Because it would still be a breaking change to drop existing public
util modules, that would be more utility modules in total, so such
a change isn't included here just for this one used-once function.)