Fix unclear IsRepositoryExist logic (#24374)

There was only one `IsRepositoryExist` function, it did: `has && isDir`

However it's not right, and it would cause 500 error when creating a new
repository if the dir exists.

Then, it was changed to `has || isDir`, it is still incorrect, it
affects the "adopt repo" logic.

To make the logic clear:

* IsRepositoryModelOrDirExist
* IsRepositoryModelExist

services/repository/adopt.go

@@ -206,7 +206,7 @@ func DeleteUnadoptedRepository(ctx context.Context, doer, u *user_model.User, re
 		}
 	}
 
-	if exist, err := repo_model.IsRepositoryExist(ctx, u, repoName); err != nil {
+	if exist, err := repo_model.IsRepositoryModelExist(ctx, u, repoName); err != nil {
 		return err
 	} else if exist {
 		return repo_model.ErrRepoAlreadyExist{