xdiff: add 'minimal' to XDF_DIFF_ALGORITHM_MASK

xdiff: add 'minimal' to XDF_DIFF_ALGORITHM_MASK

The XDF_DIFF_ALGORITHM_MASK bit mask only includes bits for the patience
and histogram diffs, not for the minimal one. This means that when
reseting the diff algorithm to the default one, one needs to separately
clear the bit for the minimal diff. There are places in the code that fail
to do that: merge-ort.c and builtin/merge-file.c.

Add the XDF_NEED_MINIMAL bit to the bit mask, and remove the separate
clearing of this bit in the places where it hasn't been forgotten.

Signed-off-by: Antonin Delpeuch <antonin@delpeuch.eu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>