cmd/go: git checkout the correct default branch.
cmd/go: git checkout the correct default branch.

origin/master is always a remote branch, and it doesn't make sense to
switch to a remote branch. master is the default branch that tracks it.

R=adg
CC=golang-dev, matt.jibson
https://codereview.appspot.com/10869046

tv42/old-go,apparentlymart/golang,pombredanne/go-deleteme,apparentlymart/golang,apparentlymart/golang,pombredanne/go-deleteme,tv42/old-go,apparentlymart/golang,tv42/old-go,tv42/old-go,apparentlymart/golang,pombredanne/go-deleteme,tv42/old-go,pombredanne/go-deleteme,tv42/old-go,tv42/old-go,apparentlymart/golang,pombredanne/go-deleteme,pombredanne/go-deleteme,apparentlymart/golang,pombredanne/go-deleteme