parse-git-describe: Don't use 'GIT' string in non-tag release

The sha abbrev in git is already prepended with a 'g' for this purpose.

Change-Id: I1915902fe875a33a5b3712f772aea109e920e0a6
Reviewed-on: http://review.couchbase.org/43499
Tested-by: Mark Nunberg <mnunberg@haskalah.org>
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>