Add branch support to repo upload

This commit adds a --br=<branch> option to repo upload.

repo currently examines every non-published branch. This is problematic
for my workflow. I have many branches in my kernel tree. Many of these
branches are based off of upstream remotes (I have many remotes) and
will never be uploaded (they'll get sent upstream as a patch).

Having repo scan these branches adds to my upload processing time
and clutters the branch selection buffer. I've also seen repo get
confused when one of my branches is 1000s of commits different from
m/master.

Change-Id: I68fa18951ea59ba373277b57ffcaf8cddd7e7a40

project.py

@@ -791,7 +791,7 @@ class Project(object):
       if R_HEADS + n not in heads:
         self.bare_git.DeleteRef(name, id)
 
-  def GetUploadableBranches(self):
+  def GetUploadableBranches(self, selected_branch=None):
     """List any branches which can be uploaded for review.
     """
     heads = {}
@@ -807,6 +807,8 @@ class Project(object):
     for branch, id in heads.iteritems():
       if branch in pubed and pubed[branch] == id:
         continue
+      if selected_branch and branch != selected_branch:
+        continue
 
       rb = self.GetUploadableBranch(branch)
       if rb: