add test coverage for original author conversion during migrations (#18506)

* add test coverage for original author conversion during migrations

And create a function to factorize a code snippet that is repeated
five times and would otherwise be more difficult to test and maintain
consistently.

Signed-off-by: Loïc Dachary <loic@dachary.org>

* fix variable scope and int64 formatting

* add missing calls to remapExternalUser and fix misplaced %d

Co-authored-by: Loïc Dachary <loic@dachary.org>
Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>
Co-authored-by: techknowlogick <techknowlogick@gitea.io>

models/user/external_login_user.go

@@ -68,6 +68,17 @@ type ExternalLoginUser struct {
 	ExpiresAt         time.Time
 }
 
+type ExternalUserMigrated interface {
+	GetExternalName() string
+	GetExternalID() int64
+}
+
+type ExternalUserRemappable interface {
+	GetUserID() int64
+	RemapExternalUser(externalName string, externalID, userID int64) error
+	ExternalUserMigrated
+}
+
 func init() {
 	db.RegisterModel(new(ExternalLoginUser))
 }