Add option to stack to use merge rather than rebase

[yashNaN]

Stacky should work just fine with either rebasing or git merging.

This makes a change that adds a [GIT] section to the stacky config and 2 settings use_merge and use_force_merge to change to merge commits and disallowing force pushes. If we disallow force pushes and want to use merging we should also disallowing amending commits, which this pr does.

Sample ~/.stackyconfig with these changes set

  [UI]
  change_to_main = True
  
  [GIT]
  use_merge = True
  use_force_push = False

Tested by creating a stack of 3 branches using normal stacky commands

[mpatou]

Looks good to me, I wished you could add some test to cover, we don't have yet too many tests but it would be good.
I would like to get the opinion of @tudor too on allowing merge.
I don't like merge we do protect against that, I also understand it's off by default but ...

[yashNaN]

Yeah I have the opposite issue, my team disallows rebasing and only allows merging. So I would need this to make stacky work with that.

I kind of buy the argument. git log --first-parent will let you see the commits like you expect. And in github you can avoid force-pushing so that when reviewing you can check only the changes since the last time you've reviewed.

[mpatou]

nit: not sure that this is really needed I guess more comments line won't hurt