Custom git command line for merge/rebase operations

We currently have no way to override any flag sent to merge/rebase during the assembly process.

For a known example, we might need to change the way fast-forward is set, but only for a specific branch. The current way --ff is set is per-project, which is not something I'm fond of. It doesn't play well with rebases and complex trees, where --ff/--no-ff in merge operations can be disruptive.

Another practical example during a merge would be to use a different merge strategy, but only for a single branch.

It is not technically hard to do. The only issue is how to specify these overrides with the current configuration format. I would love to allow the user to add "exceptions" (or any other metadata) to a rule when necessary without sacrificing the current simplicity for the common case.

Any ideas would be greatly appreciated. @guludo @erichdongubler