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>

contrib/completion/git-completion.bash

@@ -1679,7 +1679,7 @@ __git_untracked_file_modes="all no normal"
 
 __git_trailer_tokens ()
 {
-	git config --name-only --get-regexp trailer.\*.key | awk -F. '{print $2}'
+	__git config --name-only --get-regexp '^trailer\..*\.key$' | cut -d. -f 2- | rev | cut -d. -f2- | rev
 }
 
 _git_commit ()