fix(config): match ~/ gitdir includes on Windows

On Windows, `GitConfigParser` never applied an
`[includeIf "gitdir:~/..."]` section, although `git config` does.
`os.path.expanduser` returns the home directory with backslashes, so
`gitdir:~/work/` became the pattern `C:\Users\me/work/**`. GitPython
compares gitdir patterns with the repository's `git_dir` written with
forward slashes, and in a gitdir pattern a backslash escapes the next
character instead of separating path components, so the pattern could
never match.

Git converts the home directory to forward slashes when it expands `~`
on Windows (`interpolate_path()` in `path.c`, which
`prepare_include_condition_pattern()` in `config.c` uses for gitdir
conditions). A usual setup such as

    [includeIf "gitdir:~/work/"]
        path = ~/.gitconfig-work

therefore made GitPython read `user.email` and other settings for a
repository under `~/work` from the global configuration only, while Git
read them from the included file.

On Windows, a pattern that is `~` or starts with `~/` now gets the home
directory with forward slashes. The rest of the pattern is unchanged, so
a backslash written in the pattern still escapes the next character, as
`test_conditional_includes_do_not_treat_backslashes_as_separators`
expects. Other platforms and `~user` patterns keep the previous
`os.path.expanduser` call.

`test_conditional_includes_expand_home_in_git_dir_pattern` points the
home directory at a temporary directory and checks that
`gitdir:~/target/` matches a repository below it. On Windows it fails
without this change and passes with it. On other platforms it passes
either way.

Validated on Windows 11 with Git for Windows 2.51.2 and Python 3.11.9:
`test/test_config.py` passes (55 passed, 3 skipped); `ruff check`,
`ruff format --check`, `mypy` and `basedpyright --warnings` report no
issues. For a repository whose `.git/config` includes a file through
`gitdir:~/...`, `git config --get user.email` returns the included value,
and `Repo.config_reader("repository").get_value("user", "email")` now
returns it too instead of finding no value.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Author: b84330808

git/config.py

@@ -730,7 +730,12 @@ def _all_items(section: str) -> List[Tuple[str, str]]:
             value = match.group(2).strip()
 
             if keyword in ["gitdir", "gitdir/i"]:
-                value = osp.expanduser(value)
+                if sys.platform == "win32" and (value == "~" or value.startswith("~/")):
+                    # Like Git, substitute the home directory with forward slashes, as a
+                    # backslash in the pattern would escape the next character.
+                    value = osp.expanduser("~").replace("\\", "/") + value[1:]
+                else:
+                    value = osp.expanduser(value)
                 git_dir = os.fspath(self._repo.git_dir) if self._repo.git_dir else None
                 if sys.platform == "win32":
                     git_dir = git_dir.replace("\\", "/") if git_dir else None

test/test_config.py

@@ -989,6 +989,22 @@ def test_conditional_includes_do_not_treat_backslashes_as_separators(self, rw_di
         with GitConfigParser(config_path, repo=repo, merge_includes=False) as config:
             assert config._included_paths() == []
 
+    @with_rw_directory
+    def test_conditional_includes_expand_home_in_git_dir_pattern(self, rw_dir):
+        git_dir = osp.join(rw_dir, "target", "repo", ".git")
+        repo = mock.Mock(git_dir=git_dir)
+        config_path = osp.join(rw_dir, "config")
+        included_path = osp.join(rw_dir, "included")
+
+        with open(config_path, "w") as stream:
+            stream.write(f'[includeIf "gitdir:~/target/"]\n    path={included_path}\n')
+
+        # On Windows, the home directory has backslashes, but Git matches it as if it
+        # were written with forward slashes.
+        with mock.patch.dict(os.environ, {"HOME": rw_dir, "USERPROFILE": rw_dir}):
+            with GitConfigParser(config_path, repo=repo, merge_includes=False) as config:
+                assert config._included_paths() == [("path", included_path)]
+
     @with_rw_directory
     def test_conditional_includes_from_branch_name(self, rw_dir):
         # Initiate mocked branch.