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_conversion.py

@@ -23,7 +23,7 @@ def test_convert():
 
 
 def test_convert_already_worktree():
-    with TempGitRepositoryWorktree() as (git_dir, _commit):
+    with TempGitRepositoryWorktree.get(funcname()) as (git_dir, _commit):
         before = checksum_directory(git_dir)
 
         cmd = grm(["wt", "convert"], cwd=git_dir)