Reintegrate: cope with delete-modify conflicts

A more proper solution is probably to teach "rerere" to remember what modification (probably indexed via a patch ID for flexibility, but either the pre- or post-image blob ID without such flexibility is probably good enough) was deemed "uninteresting" to resolve in favor of deletion and reapply it when we see another delete-modify conflict with the same modification, but for now, treat all delete-modify conflict to be with uninteresting modification. The way the Reintegrate machinery is used makes it somewhat safer than it looks---the initial attempt to merge (and come up with a merge-fix if needed) will always be done using "git merge" and the resulting commit is manually examined (if only to see if there is a need for a further merge-fix) before redo-foo script is written out of the resulting history. We'll need to deal with a case where a modifying side need to be kept over the deleting side if/when it arises, but so far I haven't seen such a case in real life.