code review 10907043: cmd/go: fix "go get -u" for git repositories.

cmd/go: fix "go get -u" for git repositories.
CL 10869046 changed cmd/go to checkout master branch, so
for "go get -u" to work, it must "git pull" instead of
"git fetch". Added "--ff-only" so that it won't accidentally
overwrite user changes.

[minux1, 2013-07-03 09:36:55]

Hello dsymonds@golang.org (cc: golang-dev@googlegroups.com),

I'd like you to review this change to
https://code.google.com/p/go/

[dsymonds, 2013-07-03 10:31:06]

LGTM

but sit on it for a day in case there are objections.

[minux1, 2013-07-07 15:06:44]

*** Submitted as https://code.google.com/p/go/source/detail?r=fc06b300732a ***

cmd/go: fix "go get -u" for git repositories.
CL 10869046 changed cmd/go to checkout master branch, so
for "go get -u" to work, it must "git pull" instead of
"git fetch". Added "--ff-only" so that it won't accidentally
overwrite user changes.

R=dsymonds
CC=golang-dev
https://codereview.appspot.com/10907043