GitLab

return the commit hash.  Also set the `vc-git-detached' property.
(vc-git--rev-parse): Extract from `vc-git-previous-revision'.
(vc-git-mode-line-string): Use the same help-echo format whether in
detached mode or not, because we know the actual revision now.  When
in detached mode, shorten the revision to 7 chars.

Fixes: debbugs:14459