Checkout default git branch

When a git repository is in detached HEAD state use `git show remote
origin` to determine the default branch and check it out before
downloading updates.

This is necessary for `godep restore` to work since
https://github.com/golang/go/commit/42206598671a44111c8f726ad33dc7b265bdf669

Changelog.md

+#v66 (2016/06/10)
+
+* Use `git remote show origin` to find the default branch when restoring a git based package repository that is in detached head state
+
 #v65 (2016/06/09)
 
 * Rewrite update so that it considers new transitive dependencies, both in the same repo and outside of it.

restore.go

@@ -146,6 +146,22 @@ func download(dep *Dependency) error {
 
 	if !dep.vcs.exists(dep.root, dep.Rev) {
 		debugln("Updating existing", dep.root)
+		if dep.vcs == vcsGit {
+			detached, err := gitDetached(dep.root)
+			if err != nil {
+				return err
+			}
+			if detached {
+				db, err := gitDefaultBranch(dep.root)
+				if err != nil {
+					return err
+				}
+				if err := gitCheckout(dep.root, db); err != nil {
+					return err
+				}
+			}
+		}
+
 		dep.vcs.vcs.Download(dep.root)
 		downloaded[rr.Repo] = true
 	}

vcs.go

@@ -2,6 +2,7 @@ package main
 
 import (
 	"bytes"
+	"errors"
 	"fmt"
 	"os"
 	"os/exec"
@@ -274,3 +275,33 @@ func hgLink(dir, remote, url string) error {
 	fmt.Fprintf(f, "[paths]\n%s = %s\n", remote, url)
 	return f.Close()
 }
+
+func gitDetached(r string) (bool, error) {
+	o, err := vcsGit.runOutput(r, "status")
+	if err != nil {
+		return false, errors.New("unable to determine git status " + err.Error())
+	}
+	return bytes.Contains(o, []byte("HEAD detached at")), nil
+}
+
+func gitDefaultBranch(r string) (string, error) {
+	e := "Unable to determine default branch: "
+	hb := []byte("HEAD branch: ")
+	o, err := vcsGit.runOutput(r, "remote show origin")
+	if err != nil {
+		return "", errors.New(e + err.Error())
+	}
+	s := bytes.Index(o, hb)
+	if s < 0 {
+		return "", errors.New(e + "git output missing '" + string(hb) + "'")
+	}
+	f := bytes.Fields(o[s:])
+	if len(f) < 3 {
+		return "", errors.New(e + "git output too short")
+	}
+	return string(f[2]), nil
+}
+
+func gitCheckout(r, b string) error {
+	return vcsGit.run(r, "checkout "+b)
+}

version.go

@@ -8,7 +8,7 @@ import (
 	"strings"
 )
 
-const version = 65
+const version = 66
 
 var cmdVersion = &Command{
 	Name:  "version",