revision: add separate field for "-m" of "diff-index -m"

Add separate 'match_missing' field for diff-index to use and set it when we
encounter "-m" option. This field won't then be cleared when another meaning of
"-m" is reverted (e.g., by "--no-diff-merges"), nor it will be affected by
future option(s) that might drive 'ignore_merges' field.

Use this new field from diff-lib:do_oneway_diff() instead of reusing
'ignore_merges' field.

Signed-off-by: Sergey Organov <sorganov@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

revision.h

@@ -188,6 +188,7 @@ struct rev_info {
 	unsigned int	diff:1,
 			full_diff:1,
 			show_root_diff:1,
+			match_missing:1,
 			no_commit_id:1,
 			verbose_header:1,
 			combine_merges:1,