merge: add a new action type representing files to add/mark as modified This is somewhat different from the currently existing 'a' action, for the following case: - dirty working copy, with file 'fa' added and 'fm' modified - hg merge --force with a rev that neither has 'fa' nor 'fm' - for the change/delete conflicts we pick 'changed' for both 'fa' and 'fm'. In this case 'branchmerge' is true, but we need to distinguish between 'fa', which should ultimately be marked added, and 'fm', which should be marked modified. Our current strategy is to just not touch the dirstate at all. That works for now, but won't work once we move change/delete conflicts to the resolve phase. In that case we may perform repeated re-resolves, some of which might mark the file removed or remove the file from the dirstate. We'll need to re-add the file to the dirstate, and we need to be able to figure out whether we mark the file added or modified. That is what the new 'am' action lets us do.