Handle newer Git versions when checking for conflicting worktree

repoutil.Worktree is designed to clean up any existing worktree before
adding a new one. Currently, it relies on the error message produced
by Git to determine if there is one. The format of the error message
has recently changed [1], causing the test suite to fail. This fix
should work with both older and newer versions of Git.

1. git/git@2a499264d39

utility/repoutil.py

@@ -176,7 +176,10 @@ def _create_worktree(
         try:
             repo_git.worktree('add', self.worktree_dir, branch)
         except GitCommandError as e:
-            if 'is already checked out at' not in e.stderr:
+            conflicting_worktree_exists: bool = (
+                'is already checked out at' in e.stderr or 'is already used by worktree at' in e.stderr
+            )
+            if not conflicting_worktree_exists:
                 raise e
             worktrees_output: list[str] = repo.git.worktree('list', '--porcelain').splitlines()
             conflicting_worktree = None