codereview: fix for Mercurial 2.9

The branchtags method was removed but we should've
been using branchmap all along.

http://selenic.com/hg/rev/4274eda143cb

LGTM=franciscossouza, r
R=golang-codereviews, franciscossouza, r
CC=golang-codereviews
https://golang.org/cl/57500045

lib/codereview/codereview.py

@@ -2669,7 +2669,7 @@ def RietveldSetup(ui, repo):
 	rpc = None
 
 	global releaseBranch
-	tags = repo.branchtags().keys()
+	tags = repo.branchmap().keys()
 	if 'release-branch.go10' in tags:
 		# NOTE(rsc): This tags.sort is going to get the wrong
 		# answer when comparing release-branch.go9 with