On Migration respect old DefaultBranch (#12843)

* On Migration respect old DefaultBranch

* add DefaultBranch int test set

Co-authored-by: zeripath <art27@cantab.net>

modules/migrations/base/repo.go

@@ -7,13 +7,14 @@ package base
 
 // Repository defines a standard repository information
 type Repository struct {
-	Name         string
-	Owner        string
-	IsPrivate    bool
-	IsMirror     bool
-	Description  string
-	AuthUsername string
-	AuthPassword string
-	CloneURL     string
-	OriginalURL  string
+	Name          string
+	Owner         string
+	IsPrivate     bool
+	IsMirror      bool
+	Description   string
+	AuthUsername  string
+	AuthPassword  string
+	CloneURL      string
+	OriginalURL   string
+	DefaultBranch string
 }