cmd/go: fix capitalization of Bitbucket, document Git support

Fixes issue 5226.

R=golang-dev, minux.ma, iant
CC=golang-dev
https://codereview.appspot.com/8320044

Committer: Shenghou Ma <minux.ma@gmail.com>
misc: update zsh completion

R=golang-dev, minux.ma
CC=golang-dev
https://codereview.appspot.com/8437043

Committer: Shenghou Ma <minux.ma@gmail.com>