models/user: better directory handling when change username

Previously, if the user base directory somehow doesn't exist, the
application throws 500 for failure of rename.

Now it detects if the application should rename or just create a
new directory.

models/user.go

@@ -682,7 +682,13 @@ func ChangeUserName(u *User, newUserName string) (err error) {
 		return fmt.Errorf("Delete repository wiki local copy: %v", err)
 	}
 
-	return os.Rename(UserPath(u.Name), UserPath(newUserName))
+	// Rename or create user base directory
+	baseDir := UserPath(u.Name)
+	newBaseDir := UserPath(newUserName)
+	if com.IsExist(baseDir) {
+		return os.Rename(baseDir, newBaseDir)
+	}
+	return os.MkdirAll(newBaseDir, os.ModePerm)
 }
 
 func updateUser(e Engine, u *User) error {