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://golang.org/cl/10907043

src/cmd/go/vcs.go

@@ -91,7 +91,7 @@ var vcsGit = &vcsCmd{
 	cmd:  "git",
 
 	createCmd:   "clone {repo} {dir}",
-	downloadCmd: "fetch",
+	downloadCmd: "pull --ff-only",
 
 	tagCmd: []tagCmd{
 		// tags/xxx matches a git tag named xxx