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://golang.org/cl/10869046

src/cmd/go/vcs.go

@@ -102,7 +102,7 @@ var vcsGit = &vcsCmd{
 		{"show-ref tags/{tag} origin/{tag}", `((?:tags|origin)/\S+)$`},
 	},
 	tagSyncCmd:     "checkout {tag}",
-	tagSyncDefault: "checkout origin/master",
+	tagSyncDefault: "checkout master",
 
 	scheme:  []string{"git", "https", "http", "git+ssh"},
 	pingCmd: "ls-remote {scheme}://{repo}",