Add a test for git.index.util.TemporaryFileSwap

This is a general test for TemporaryFileSwap, but by being
parametrized by the type of file_path, it reveals a regression
introduced in 9e86053 (gitpython-developers#1770). TemporaryFileSwap still works when
file_path is a string, but is now broken when it is a Path. That
worked before, and the type annotations document that it should
be able to work. This is at least a bug because TemporaryFileSwap
is public. (I am unsure whether, in practice, GitPython itself uses
it in a way that sometimes passes a Path object as file_path. But
code that uses GitPython may call it directly and pass Path.)

test/test_index.py

@@ -34,10 +34,11 @@
 )
 from git.index.fun import hook_path
 from git.index.typ import BaseIndexEntry, IndexEntry
+from git.index.util import TemporaryFileSwap
 from git.objects import Blob
-from test.lib import TestBase, fixture, fixture_path, with_rw_directory, with_rw_repo
 from git.util import Actor, hex_to_bin, rmtree
 from gitdb.base import IStream
+from test.lib import TestBase, fixture, fixture_path, with_rw_directory, with_rw_repo
 
 HOOKS_SHEBANG = "#!/usr/bin/env sh\n"
 
@@ -1087,3 +1088,25 @@ def test_index_add_pathlike(self, rw_repo):
         file.touch()
 
         rw_repo.index.add(file)
+
+
+class TestIndexUtils:
+    @pytest.mark.parametrize("file_path_type", [str, Path])
+    def test_temporary_file_swap(self, tmp_path, file_path_type):
+        file_path = tmp_path / "foo"
+        file_path.write_bytes(b"some data")
+
+        with TemporaryFileSwap(file_path_type(file_path)) as ctx:
+            assert Path(ctx.file_path) == file_path
+            assert not file_path.exists()
+
+            # Recreate it with new data, so we can observe that they're really separate.
+            file_path.write_bytes(b"other data")
+
+            temp_file_path = Path(ctx.tmp_file_path)
+            assert temp_file_path.parent == file_path.parent
+            assert temp_file_path.name.startswith(file_path.name)
+            assert temp_file_path.read_bytes() == b"some data"
+
+        assert not temp_file_path.exists()
+        assert file_path.read_bytes() == b"some data"  # Not b"other data".