If revision is sha hash and dest-branch is defined, use it for starting branch

Change-Id: I538c7d216f72b87629b61aee547d374a398c95da

subcmds/start.py

@@ -59,9 +59,13 @@ revision specified in the manifest.
     for project in all_projects:
       pm.update()
       # If the current revision is a specific SHA1 then we can't push back
-      # to it so substitute the manifest default revision instead.
+      # to it; so substitute with dest_branch if defined, or with manifest
+      # default revision instead.
       if IsId(project.revisionExpr):
-        project.revisionExpr = self.manifest.default.revisionExpr
+        if project.dest_branch:
+          project.revisionExpr = project.dest_branch
+        else:
+          project.revisionExpr = self.manifest.default.revisionExpr
       if not project.StartBranch(nb):
         err.append(project)
     pm.end()