feature request: allow disabling merges for certain repos

[connoryates]

We have a very busy GitOps repo where the rebase feature of marge-bot is very useful to us. However, we want the original authors to own merging since merge = deployment in this repo. We've patched our own internal version of marge-bot to include a --no-merge option which a) does not merge the MR and b) does not unassign marge-bot from the MR.

I realize this essentially turns marge-bot into a rebase bot, but I think this would be a nice feature to include. There are some repos where we would love to use merging features with