When starting a branch, branch.merge is set to project revision unless
the revision is a SHA1. In that case, branch.merge is set to dest_branch
if defined or manifest default revision otherwise. This special handling
allows repo upload to work when the project revision is a SHA1.
Extend the special handling to also happen when the project revision
is a tag value or a change value so that repo upload will work in those
case as well.
Change-Id: Iff81ece40e770cd02535e80dcb023564d42dcf47