forall: Don't try to get lrev of projects in mirror workspace

git rev-parse fails for projects that don't have an explicit revision
specified, and don't have a branch of the same name as the default
revision. This can be the case in a workspace synced with the smart
sync (-s) or smart tag (-t) option.

Change-Id: I19bfe9fe7396170379415d85f10f6440dc6ea08f