Use branch as the initial constraint if specified in gb vendor manifest

cmd/dep/gb_importer.go

@@ -139,11 +139,17 @@ func (i *gbImporter) convertOne(pkg gbDependency) (pc gps.ProjectConstraint, lp
 
 	pc.Ident = gps.ProjectIdentifier{ProjectRoot: gps.ProjectRoot(pkg.Importpath), Source: pkg.Repository}
 
-	// The default constraint is just a revision
+	// Generally, gb tracks revisions
 	var revision = gps.Revision(pkg.Revision)
 
+	// But if the branch field is not "HEAD", we can use that as the initial constraint
+	var constraint gps.Constraint
+	if pkg.Branch != "HEAD" {
+		constraint = gps.NewBranch(pkg.Branch)
+	}
+
 	// See if we can get a version from that constraint
-	version, err := lookupVersionForLockedProject(pc.Ident, nil, revision, i.sm)
+	version, err := lookupVersionForLockedProject(pc.Ident, constraint, revision, i.sm)
 	if err != nil {
 		// Log the error, but don't fail it. It's okay if we can't find a version
 		i.logger.Println(err.Error())