Improve handling of git remote show origin

This should help in cases where remote HEAD is ambiguous.
See the new tests. If we run across any other/different output we can
add it to the tests.

vcs.go

@@ -285,21 +285,36 @@ func gitDetached(r string) (bool, error) {
 }
 
 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())
+		return "", errors.New("Running git remote show origin errored with: " + err.Error())
 	}
-	s := bytes.Index(o, hb)
+	return gitDetermineDefaultBranch(r, string(o))
+}
+
+func gitDetermineDefaultBranch(r, o string) (string, error) {
+	e := "Unable to determine HEAD branch: "
+	hb := "HEAD branch:"
+	lbcfgp := "Local branch configured for 'git pull':"
+	s := strings.Index(o, hb)
 	if s < 0 {
-		return "", errors.New(e + "git output missing '" + string(hb) + "'")
+		b := strings.Index(o, lbcfgp)
+		if b < 0 {
+			return "", errors.New(e + "Remote HEAD is ambiguous. Before godep can pull new commits you will need to:" + `
+cd ` + r + `
+git checkout <a HEAD branch>
+Here is what was reported:
+` + o)
+		}
+		s = b + len(lbcfgp)
+	} else {
+		s += len(hb)
 	}
-	f := bytes.Fields(o[s:])
+	f := strings.Fields(o[s:])
 	if len(f) < 3 {
 		return "", errors.New(e + "git output too short")
 	}
-	return string(f[2]), nil
+	return f[0], nil
 }
 
 func gitCheckout(r, b string) error {

vcs_test.go

@@ -0,0 +1,94 @@
+package main
+
+import "testing"
+
+func TestGitDetermineDefaultBranch(t *testing.T) {
+	cases := []struct {
+		r, o string
+		v    string
+		err  bool
+	}{
+		{"test",
+			`* remote origin
+  Fetch URL: https://gopkg.in/mgo.v2
+  Push  URL: https://gopkg.in/mgo.v2
+  HEAD branch: v2
+  Remote branches:
+    master      tracked
+    v2          tracked
+    v2-unstable tracked
+  Local branches configured for 'git pull':
+    master merges with remote master
+    v2     merges with remote v2
+  Local refs configured for 'git push':
+    master pushes to master (up to date)
+    v2     pushes to v2     (local out of date)
+`, "v2", false},
+		{"test",
+			`* remote origin
+  Fetch URL: https://gopkg.in/bluesuncorp/validator.v5
+  Push  URL: https://gopkg.in/bluesuncorp/validator.v5
+  HEAD branch (remote HEAD is ambiguous, may be one of the following):
+    master
+    v5
+  Remote branches:
+    krhubert       tracked
+    master         tracked
+    v4             tracked
+    v5             tracked
+    v5-development tracked
+    v6             tracked
+    v6-development tracked
+    v7             tracked
+    v7-development tracked
+    v8             tracked
+    v8-development tracked
+  Local branch configured for 'git pull':
+    master merges with remote master
+  Local ref configured for 'git push':
+    master pushes to master (up to date)
+`, "master", false},
+		{"test",
+			`* remote origin
+  Fetch URL: https://github.com/gin-gonic/gin
+  Push  URL: https://github.com/gin-gonic/gin
+  HEAD branch: develop
+  Remote branches:
+    benchmarks            tracked
+    better-bind-errors    tracked
+    develop               tracked
+    fasthttp              tracked
+    fix-binding           tracked
+    fix-tests             tracked
+    gh-pages              tracked
+    honteng-bind_test     tracked
+    master                tracked
+    new-binding-validator tracked
+    new-catch-all         tracked
+    performance           tracked
+    routes-list           tracked
+  Local branch configured for 'git pull':
+    develop merges with remote develop
+  Local ref configured for 'git push':
+    develop pushes to develop (local out of date)
+`, "develop", false},
+		{"test", "", "", true},
+	}
+
+	for i, test := range cases {
+		v, e := gitDetermineDefaultBranch(test.r, test.o)
+		if v != test.v {
+			t.Errorf("%d Unexpected value returned: %s, wanted %s", i, v, test.v)
+		}
+		if e != nil {
+			t.Log("Err", e.Error())
+		}
+		if test.err && e == nil {
+			t.Errorf("%d Test should err, but didn't", i)
+		}
+
+		if !test.err && e != nil {
+			t.Errorf("%d Test shouldn't err, but did with: %s", i, e)
+		}
+	}
+}