codereview: fix for Mercurial 2.9
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://codereview.appspot.com/57500045

apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang