Add a missing PermissionError xfail on Windows

One of the tests that was commented as being skipped as a result of
SkipTest rasied in git.util.rmtree or one of the functions that
calls it, test_git_submodule_compatibility, was not skipped in that
way and was actually failing on Windows with PermissionError. It
appears that the cause of failure changed over time, so that it
once involved rmtree but no longer does.

This removes the outdated comment and adds an xfail mark instead,
specific to PermissionError and with a message identifying where in
the test case's logic the PermissionError is currently triggered.

Author: EliahKagan

Diff for: test/test_submodule.py

@@ -819,9 +819,11 @@ def test_git_submodules_and_add_sm_with_new_commit(self, rwdir):
         assert commit_sm.binsha == sm_too.binsha
         assert sm_too.binsha != sm.binsha
 
-    # @skipIf(HIDE_WINDOWS_KNOWN_ERRORS,  ## ACTUALLY skipped by `git.submodule.base#L869`.
-    #         "FIXME: helper.wrapper fails with: PermissionError: [WinError 5] Access is denied: "
-    #         "'C:\\Users\\appveyor\\AppData\\Local\\Temp\\1\\test_work_tree_unsupportedryfa60di\\master_repo\\.git\\objects\\pack\\pack-bc9e0787aef9f69e1591ef38ea0a6f566ec66fe3.idx")  # noqa E501
+    @pytest.mark.xfail(
+        HIDE_WINDOWS_KNOWN_ERRORS,
+        reason='"The process cannot access the file because it is being used by another process" on call to sm.move',
+        raises=PermissionError,
+    )
     @with_rw_directory
     def test_git_submodule_compatibility(self, rwdir):
         parent = git.Repo.init(osp.join(rwdir, "parent"))