Decrement Fork Num when converting from Fork (#17035) (#17046)

Backport #17035

When converting repositories from forks to normal the root NumFork needs to be
decremented too.

Fix #17026

Signed-off-by: Andrew Thornton <art27@cantab.net>

routers/web/repo/setting.go

@@ -538,10 +538,8 @@ func SettingsPost(ctx *context.Context) {
 			return
 		}
 
-		repo.IsFork = false
-		repo.ForkID = 0
-		if err := models.UpdateRepository(repo, false); err != nil {
-			log.Error("Unable to update repository %-v whilst converting from fork", repo)
+		if err := repository.ConvertForkToNormalRepository(repo); err != nil {
+			log.Error("Unable to convert repository %-v from fork. Error: %v", repo, err)
 			ctx.ServerError("Convert Fork", err)
 			return
 		}