e2e: Add caching to git repositories

It's very expensive to create new repositories from scratch. To avoid
this, a new repo & remotes are only created if necessary (depending on a
cache key given on request). If not created, they are simply copied from
a stored, clean repository / remote.

e2e_tests/test_worktree_rebase.py

@@ -14,7 +14,7 @@ import git
 @pytest.mark.parametrize("has_changes", [True, False])
 @pytest.mark.parametrize("stash", [True, False])
 def test_worktree_rebase(pull, rebase, ffable, has_changes, stash):
-    with TempGitRepositoryWorktree() as (base_dir, _root_commit):
+    with TempGitRepositoryWorktree.get(funcname()) as (base_dir, _root_commit):
         with open(os.path.join(base_dir, "grm.toml"), "w") as f:
             f.write('persistent_branches = ["mybasebranch"]')