merge-file: add --diff-algorithm option

merge-file: add --diff-algorithm option

Make it possible to use other diff algorithms than the 'myers'
default algorithm, when using the 'git merge-file' command, to help
avoid spurious conflicts by selecting a more recent algorithm such
as 'histogram', for instance when using 'git merge-file' as part of
a custom merge driver.

Signed-off-by: Antonin Delpeuch <antonin@delpeuch.eu>
Reviewed-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>