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_fetch.py

@@ -9,7 +9,7 @@ import git
 
 
 def test_worktree_fetch():
-    with TempGitRepositoryWorktree() as (base_dir, root_commit):
+    with TempGitRepositoryWorktree.get(funcname()) as (base_dir, root_commit):
         with TempGitFileRemote() as (remote_path, _remote_sha):
             shell(
                 f"""
@@ -56,7 +56,7 @@ def test_worktree_fetch():
 @pytest.mark.parametrize("has_changes", [True, False])
 @pytest.mark.parametrize("stash", [True, False])
 def test_worktree_pull(rebase, ffable, has_changes, stash):
-    with TempGitRepositoryWorktree() as (base_dir, root_commit):
+    with TempGitRepositoryWorktree.get(funcname()) as (base_dir, root_commit):
         with TempGitFileRemote() as (remote_path, _remote_sha):
             shell(
                 f"""