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

subcmds/forall.py

@@ -151,11 +151,15 @@ without iterating through the remaining projects.
     attributes that we need.
 
     """
+    if not self.manifest.IsMirror:
+      lrev = project.GetRevisionId()
+    else:
+      lrev = None
     return {
       'name': project.name,
       'relpath': project.relpath,
       'remote_name': project.remote.name,
-      'lrev': project.GetRevisionId(),
+      'lrev': lrev,
       'rrev': project.revisionExpr,
       'annotations': dict((a.name, a.value) for a in project.annotations),
       'gitdir': project.gitdir,