This makes no changes to current code, but it allows the individual merge
functions to decide what to do about the old entry.  They might decide to
update it in place, rather than force them to always delete and re-add it.
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>