Reintegrate: cope with delete-modify conflicts

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.