vc-diff does not work in tramp buffer

### What did you expect to happen?

Run `M-x vc-diff` in a tramp buffer visiting a remote file, it displays the difference of working file and file in repository.

### What actually happened?

Emacs prompts "File is not under version control" in mode-line and nothing happens.

### Describe your attempts to resolve the issue

Try to reset `vc-ignore-dir-regexp` to its original value, `M-x vc-diff` works again.

commit 0b5243c12c1e69787b99f71a7c7c729ea15dd1a1 is related to the modification of `vc-ignore-dir-regexp`.

### Steps to reproduce

1. Open emacs
2. Open a tramp file in a git repository: ssh:user@host:./file.el
3. Do some modification to that file
4. `M-x vc-diff` to find difference.

### System Information

https://pastebin.com/xG0hYBKw