make-release: update local repositories

Whenever using --git-root to point to a local repository, we might be
using an out of date local clone. Hence, whenever the remote repository
is a local directory, call in there 'git remote update' to fetch from
Gerrit the branches/tags/objects.

Change-Id: Ibb9e7804dcf5162a293f0cdb9057236d0a7aa8ac