cmd/go: "go get" don't ignore git default branch

Any Git branch can be the default branch not only master. Removing
hardwired 'checkout master', and using 'checkout {tag}' is the best
choice. It works with and without a master branch. Furthermore it
resolves the Github default branch issue. Changing Github default
branch is effectively changing HEAD.

Fixes #9032

Change-Id: I19a1221bcefe0806e7556c124c6da7ac0c2160b5
Reviewed-on: https://go-review.googlesource.com/5312Reviewed-by: Russ Cox <rsc@golang.org>

src/cmd/go/vcs.go

@@ -115,8 +115,12 @@ var vcsGit = &vcsCmd{
 	tagLookupCmd: []tagCmd{
 		{"show-ref tags/{tag} origin/{tag}", `((?:tags|origin)/\S+)$`},
 	},
-	tagSyncCmd:     "checkout {tag}",
-	tagSyncDefault: "checkout master",
+	tagSyncCmd: "checkout {tag}",
+	// both createCmd and downloadCmd update the working dir.
+	// No need to do more here. We used to 'checkout master'
+	// but that doesn't work if the default branch is not named master.
+	// See golang.org/issue/9032.
+	tagSyncDefault: "",
 
 	scheme:     []string{"git", "https", "http", "git+ssh"},
 	pingCmd:    "ls-remote {scheme}://{repo}",