Aggressive three-way merge: fix D/F case

Aggressive three-way merge: fix D/F case

When the ancestor used to have a blob "P", your tree removed it, and the
tree you are merging with also removed it, the agressive three-way cleanly
merges to remove that blob.  If the other tree added a new blob "P/Q"
while removing "P", it should also merge cleanly to remove "P" and create
"P/Q" (since neither the ancestor nor your tree could have had it, so it
is a typical "created in one").

The "aggressive" rule is not new anymore.  Reword the stale comment.

Signed-off-by: Junio C Hamano <gitster@pobox.com>