Add `Irmin.try_merge_*` functions which do not update the current head

[samoht]

It would also be useful to get the commit object from the merge before updating the branch. That way, I could try loading it as a sanity check (and thus reject any update that would result in an unreadable state). I guess I need to create another temporary branch, merge to that, then force-update master to it. Seems like a lot of work, though.