Allow resolving conflicts when squashing

Once squashing is added (https://gitlab.com/gitlab-org/gitlab-ee/issues/150), we can allow resolving conflicts when squashing.

We can't do this for the merge methods that involve rebasing, because a rebase can have:

1. Conflicts that don't exist when merging. (If commit A adds a conflict, and commit B removes it, then there will still be a conflict during the rebase.)
2. Multiple sets of conflicts. (If both commits A and B have conflicts, this is an interactive process, and that's not a natural fit for a web site.)

A squash has neither of those things! So we could have a 'you've got conflicts, so you can't rebase, but you can squash and solve the conflicts at once if you choose' option. Obviously it could be named better 🙂