Send reviews to a different branch from fetch

This adds the ability to have reviews pushed to a different branch
from the one on which changes are based. This is useful for "gateway"
systems without smartsync.

Change-Id: I3a8a0fabcaf6055e62d3fb55f89c944e2f81569f

manifest_xml.py

@@ -555,6 +555,8 @@ class XmlManifest(object):
     if d.revisionExpr == '':
       d.revisionExpr = None
 
+    d.destBranchExpr = node.getAttribute('dest-branch') or None
+
     sync_j = node.getAttribute('sync-j')
     if sync_j == '' or sync_j is None:
       d.sync_j = 1
@@ -676,6 +678,8 @@ class XmlManifest(object):
         raise ManifestParseError('invalid clone-depth %s in %s' %
                                  (clone_depth, self.manifestFile))
 
+    dest_branch = node.getAttribute('dest-branch') or self._default.destBranchExpr
+
     upstream = node.getAttribute('upstream')
 
     groups = ''
@@ -709,7 +713,8 @@ class XmlManifest(object):
                       sync_s = sync_s,
                       clone_depth = clone_depth,
                       upstream = upstream,
-                      parent = parent)
+                      parent = parent,
+                      dest_branch = dest_branch)
 
     for n in node.childNodes:
       if n.nodeName == 'copyfile':