Use a fallback mergetool to resolve easy conflicts

[ChristophWurst]

While manually backporting nextcloud/documentation#11565 I notice that git can't automatically cherry-pick the full changes to stable27. Locally I use kdiff3 as mergetool and running "git mergetool" resolves conflicts without my help. So it would be quite cool if the bot could resolve those as well. We might just want to show a warning that there were conflicts and those were resolved automatically.