Add support for source-repos gerrit refs

Add the ability to supply a gerrit ref in DIB_REPOREF_* and
have that reference fetched from the remote and checked out for
building within the image.

Closes-Bug: #1314021
Change-Id: I5e5742c4cbff98e14121c50dde5e512f192b5415
Co-Authored-By: Robert Collins <rbtcollins@hp.com>