Add check for new path existence

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/26641

There are a few options here including adding the user ID if the new path / route exists. I went for just reusing the current rename_path loop. My thinking was that if I was an affected user, and saw my project being renamed from test.git to test_9324824 I'll probably have to rename it back to test_git or test_git0 if the first existed already.