When starting a branch, do not use a tag or change value for branch.merge

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

git_refs.py

@@ -16,11 +16,12 @@
 import os
 from trace import Trace
 
-HEAD    = 'HEAD'
-R_HEADS = 'refs/heads/'
-R_TAGS  = 'refs/tags/'
-R_PUB   = 'refs/published/'
-R_M     = 'refs/remotes/m/'
+HEAD      = 'HEAD'
+R_CHANGES = 'refs/changes/'
+R_HEADS   = 'refs/heads/'
+R_TAGS    = 'refs/tags/'
+R_PUB     = 'refs/published/'
+R_M       = 'refs/remotes/m/'
 
 
 class GitRefs(object):