cmd/go: do not force use of git master branch (again)

This time with a test.
Also adjust another test to skip when hg is not present,
and delete no longer needed fixDetachedHead code.

Fixes #9032 (again).

Change-Id: I481717409e1d44b524f83c70a8dc377699d1a2a5
Reviewed-on: https://go-review.googlesource.com/18334
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/cmd/go/go_test.go

@@ -1063,7 +1063,6 @@ func TestImportCommentConflict(t *testing.T) {
 // cmd/go: custom import path checking should not apply to github.com/xxx/yyy.
 func TestIssue10952(t *testing.T) {
 	testenv.MustHaveExternalNetwork(t)
-
 	if _, err := exec.LookPath("git"); err != nil {
 		t.Skip("skipping because git binary not found")
 	}
@@ -1081,6 +1080,34 @@ func TestIssue10952(t *testing.T) {
 	tg.run("get", "-d", "-u", importPath)
 }
 
+func TestGetGitDefaultBranch(t *testing.T) {
+	testenv.MustHaveExternalNetwork(t)
+	if _, err := exec.LookPath("git"); err != nil {
+		t.Skip("skipping because git binary not found")
+	}
+
+	tg := testgo(t)
+	defer tg.cleanup()
+	tg.parallel()
+	tg.tempDir("src")
+	tg.setenv("GOPATH", tg.path("."))
+
+	// This repo has two branches, master and another-branch.
+	// The another-branch is the default that you get from 'git clone'.
+	// The go get command variants should not override this.
+	const importPath = "github.com/rsc/go-get-default-branch"
+
+	tg.run("get", "-d", importPath)
+	repoDir := tg.path("src/" + importPath)
+	defer tg.resetReadOnlyFlagAll(repoDir)
+	tg.runGit(repoDir, "branch", "--contains", "HEAD")
+	tg.grepStdout(`\* another-branch`, "not on correct default branch")
+
+	tg.run("get", "-d", "-u", importPath)
+	tg.runGit(repoDir, "branch", "--contains", "HEAD")
+	tg.grepStdout(`\* another-branch`, "not on correct default branch")
+}
+
 func TestDisallowedCSourceFiles(t *testing.T) {
 	tg := testgo(t)
 	defer tg.cleanup()
@@ -2219,6 +2246,9 @@ func TestGoGetInsecureCustomDomain(t *testing.T) {
 
 func TestIssue10193(t *testing.T) {
 	testenv.MustHaveExternalNetwork(t)
+	if _, err := exec.LookPath("hg"); err != nil {
+		t.Skip("skipping because hg binary not found")
+	}
 
 	tg := testgo(t)
 	defer tg.cleanup()

src/cmd/go/vcs.go

@@ -137,8 +137,9 @@ var vcsGit = &vcsCmd{
 	// 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.
+	// DO NOT add 'checkout master' here.
 	// See golang.org/issue/9032.
-	tagSyncDefault: []string{"checkout master", "submodule update --init --recursive"},
+	tagSyncDefault: []string{"submodule update --init --recursive"},
 
 	scheme:     []string{"git", "https", "http", "git+ssh", "ssh"},
 	pingCmd:    "ls-remote {scheme}://{repo}",
@@ -385,9 +386,6 @@ func (v *vcsCmd) create(dir, repo string) error {
 
 // download downloads any new changes for the repo in dir.
 func (v *vcsCmd) download(dir string) error {
-	if err := v.fixDetachedHead(dir); err != nil {
-		return err
-	}
 	for _, cmd := range v.downloadCmd {
 		if !go15VendorExperiment && strings.Contains(cmd, "submodule") {
 			continue
@@ -399,30 +397,6 @@ func (v *vcsCmd) download(dir string) error {
 	return nil
 }
 
-// fixDetachedHead switches a Git repository in dir from a detached head to the master branch.
-// Go versions before 1.2 downloaded Git repositories in an unfortunate way
-// that resulted in the working tree state being on a detached head.
-// That meant the repository was not usable for normal Git operations.
-// Go 1.2 fixed that, but we can't pull into a detached head, so if this is
-// a Git repository we check for being on a detached head and switch to the
-// real branch, almost always called "master".
-// TODO(dsymonds): Consider removing this for Go 1.3.
-func (v *vcsCmd) fixDetachedHead(dir string) error {
-	if v != vcsGit {
-		return nil
-	}
-
-	// "git symbolic-ref HEAD" succeeds iff we are not on a detached head.
-	if err := v.runVerboseOnly(dir, "symbolic-ref HEAD"); err == nil {
-		// not on a detached head
-		return nil
-	}
-	if buildV {
-		log.Printf("%s on detached head; repairing", dir)
-	}
-	return v.run(dir, "checkout master")
-}
-
 // tags returns the list of available tags for the repo in dir.
 func (v *vcsCmd) tags(dir string) ([]string, error) {
 	var tags []string