diff: squelch empty diffs even more

When we compare two non-tracked files, or explicitly
specify --no-index, the suggestion to run git-status
is not helpful.

The patch adds a new diff_options bitfield member, no_index, that
is used instead of the special value of -2 of the rev_info field
max_count to indicate that the index is not to be used.  This makes
it possible to pass that flag down to diffcore_skip_stat_unmatch(),
which only has one diff_options parameter.

This could even become a cleanup if we removed all assignments of
max_count to a value of -2 (viz. replacement of a magic value with
a self-documenting field name) but I didn't dare to do that so late
in the rc game..

The no_index bit, if set, then tells diffcore_skip_stat_unmatch()
to not account for any skipped stat-mismatches, which avoids the
suggestion to run git-status.

Signed-off-by: Rene Scharfe <rene.scharfe@lsfire.ath.cx>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

diff-lib.c

@@ -189,6 +189,7 @@ static int handle_diff_files_args(struct rev_info *revs,
 				!strcmp(argv[1], "--no-index")) {
 			revs->max_count = -2;
 			revs->diffopt.exit_with_status = 1;
+			revs->diffopt.no_index = 1;
 		}
 		else if (!strcmp(argv[1], "-q"))
 			*silent = 1;
@@ -204,8 +205,10 @@ static int handle_diff_files_args(struct rev_info *revs,
 		 */
 		read_cache();
 		if (!is_in_index(revs->diffopt.paths[0]) ||
-					!is_in_index(revs->diffopt.paths[1]))
+					!is_in_index(revs->diffopt.paths[1])) {
 			revs->max_count = -2;
+			revs->diffopt.no_index = 1;
+		}
 	}
 
 	/*
@@ -293,6 +296,7 @@ int setup_diff_no_index(struct rev_info *revs,
 	else
 		revs->diffopt.paths = argv + argc - 2;
 	revs->diffopt.nr_paths = 2;
+	revs->diffopt.no_index = 1;
 	revs->max_count = -2;
 	return 0;
 }
@@ -304,7 +308,7 @@ int run_diff_files_cmd(struct rev_info *revs, int argc, const char **argv)
 	if (handle_diff_files_args(revs, argc, argv, &silent_on_removed))
 		return -1;
 
-	if (revs->max_count == -2) {
+	if (revs->diffopt.no_index) {
 		if (revs->diffopt.nr_paths != 2)
 			return error("need two files/directories with --no-index");
 		if (queue_diff(&revs->diffopt, revs->diffopt.paths[0],

diff.c

@@ -3185,6 +3185,7 @@ static void diffcore_skip_stat_unmatch(struct diff_options *diffopt)
 			 * to determine how many paths were dirty only
 			 * due to stat info mismatch.
 			 */
+			if (!diffopt->no_index)
 				diffopt->skip_stat_unmatch++;
 			diff_free_filepair(p);
 		}

diff.h

@@ -60,6 +60,7 @@ struct diff_options {
 		 color_diff_words:1,
 		 has_changes:1,
 		 quiet:1,
+		 no_index:1,
 		 allow_external:1,
 		 exit_with_status:1;
 	int context;