merge-trees: let caller decide whether to renormalize

merge-trees: let caller decide whether to renormalize

Add a "renormalize" option to struct merge_options so callers can
decide on a case-by-case basis whether the merge is likely to have
overlapped with a change in smudge/clean rules.  The option defaults
to the global merge_renormalize setting for now.

No change in behavior intended.

Cc: Eyvind Bernhardsen <eyvind.bernhardsen@gmail.com>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>