goinstall: Use 'git pull' instead of 'git checkout' when updating a git repo.
goinstall: Use 'git pull' instead of 'git checkout' when updating a git repo.

R=rsc
CC=golang-dev
http://codereview.appspot.com/1395041

Committer: Russ Cox <5ad239cb8a44f659eaaee0aa1ea5b94947abe557@golang.org>

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