completion: commit: complete trailers tokens more robustly

completion: commit: complete trailers tokens more robustly

In the previous commit, we added support for completing configured
trailer tokens in 'git commit --trailer'.

Make the implementation more robust by:

- using '__git' instead of plain 'git', as the rest of the completion
  script does
- using a stricter pattern for --get-regexp to avoid false hits
- using 'cut' and 'rev' instead of 'awk' to account for tokens including
  dots.

Signed-off-by: Philippe Blain <levraiphilippeblain@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>