codereview: fix submit of merge + include branch prefix in submitted message
codereview: fix submit of merge + include branch prefix in submitted message

hg insists that we not list explicit files for a merge.

for benefit of other tools reading logs, include branch prefix
at start of every commit message.

LGTM=minux
R=minux
CC=golang-codereviews
https://codereview.appspot.com/124780044

wesolows/golang,wesolows/golang,wesolows/golang-old,firebitsbr/golang-old,wesolows/golang-old,wesolows/golang-old,wesolows/golang,firebitsbr/golang-old,wesolows/golang,wesolows/golang-old,firebitsbr/golang-old,wesolows/golang-old,firebitsbr/golang-old,wesolows/golang-old,firebitsbr/golang-old,firebitsbr/golang-old,firebitsbr/golang-old,wesolows/golang,wesolows/golang,wesolows/golang-old,wesolows/golang,firebitsbr/golang-old