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>

modules/migration/comment.go

@@ -18,3 +18,9 @@ type Comment struct {
 	Content     string
 	Reactions   []*Reaction
 }
+
+// GetExternalName ExternalUserMigrated interface
+func (c *Comment) GetExternalName() string { return c.PosterName }
+
+// ExternalID ExternalUserMigrated interface
+func (c *Comment) GetExternalID() int64 { return c.PosterID }