Maintain fully qualified tracking branches

When running repo branch, the git merge line (in many circumstances)
is set to the revision of the project specified in the manifest.  If
this is a branch name that is not fully-qualified, we will end up with
something like "merge = master" instead of "merge = refs/heads/master".
This change examines the revision if we are going to use that and
changes branch short names to fully qualified branch names.

Change-Id: Ie1be94fb8d45df8eeac44a47f729a3819a05fa81

project.py

@@ -1326,6 +1326,8 @@ class Project(object):
     if not ID_RE.match(self.revisionExpr):
       # in case of manifest sync the revisionExpr might be a SHA1
       branch.merge = self.revisionExpr
+      if not branch.merge.startswith('refs/'):
+        branch.merge = R_HEADS + branch.merge
     branch.Save()
 
     if cnt_mine > 0 and self.rebase:
@@ -1394,6 +1396,8 @@ class Project(object):
     branch = self.GetBranch(name)
     branch.remote = self.GetRemote(self.remote.name)
     branch.merge = self.revisionExpr
+    if not branch.merge.startswith('refs/'):
+      branch.merge = R_HEADS + self.revisionExpr
     revid = self.GetRevisionId(all_refs)
 
     if head.startswith(R_HEADS):