diff-merges: new function diff_merges_suppress()

This function sets all the relevant flags to disabled state, so that
no code that checks only one of them get it wrong.

Then we call this new function everywhere where diff merges output
suppression is needed.

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

fmt-merge-msg.c

@@ -2,6 +2,7 @@
 #include "refs.h"
 #include "object-store.h"
 #include "diff.h"
+#include "diff-merges.h"
 #include "revision.h"
 #include "tag.h"
 #include "string-list.h"
@@ -668,7 +669,7 @@ int fmt_merge_msg(struct strbuf *in, struct strbuf *out,
 		head = lookup_commit_or_die(&head_oid, "HEAD");
 		repo_init_revisions(the_repository, &rev, NULL);
 		rev.commit_format = CMIT_FMT_ONELINE;
-		rev.ignore_merges = 1;
+		diff_merges_suppress(&rev);
 		rev.limited = 1;
 
 		strbuf_complete_line(out);