merge bot: allow to squash commits of the PR

[pedrobaeza]

Include a command for squashing the commits of the pull request independently from the fixup! message in them (not everyone makes this properly).

[sbidoul]

Add an explicit option in ocabot merge command to request commit squashing.

Two options for the commit message:

1. use the PR title + description (my preference)
2. concatenate all commit messages

[simahawk]

would be nice to support:

• autosquash - only for fixup! commits, preserve messages (eg: migration PR)
• squash - squash them all, use PR title as main message

[guewen]

Maybe as a first step, could we have the bot refuses to merge if it finds any commit with fixup! and asking the contributor to squash? It could prevent having fixup commits merged such as in OCA/wms#32

[yajo]

@sbidoul a kind remainder about this issue. According to your last movements trying to improve the landing experience of functional people writing docs, this is gonna be a problem. They will probably not write correct commit message, or understand why they should (actually this happens with many devs too, after all...).

Maybe fixing this issue should land in your list of boxes to tick for that goal.