DeleteRepoFile incorrectly handles Delete to new branch

[zeripath]

Whilst looking at #9708 I've noticed that there is an issue with:

gitea/modules/repofiles/delete.go

Line 49 in 1751d5f

if git.IsErrNotExist(err) {

This should be

if err != nil && !git.IsErrNotExist(err) {

however this breaks several tests which need to also be fixed.