diff --quiet: disable optimization when --diff-filter=X is used

The code notices that the caller does not want any detail of the changes
and only wants to know if there is a change or not by specifying --quiet.
And it breaks out of the loop when it knows it already found any change.

When you have a post-process filter (e.g. --diff-filter), however, the
path we found to be different in the previous round and set HAS_CHANGES
bit may end up being uninteresting, and there may be no output at the end.
The optimization needs to be disabled for such case.

Note that the f245194 (diff: change semantics of "ignore whitespace"
options, 2009-05-22) already disables this optimization by refraining
from setting HAS_CHANGES when post-process filters that need to inspect
the contents of the files (e.g. -S, -w) in diff_change() function.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

diff-lib.c

@@ -74,6 +74,7 @@ int run_diff_files(struct rev_info *revs, unsigned int option)
 		int changed;
 
 		if (DIFF_OPT_TST(&revs->diffopt, QUICK) &&
+		    !revs->diffopt.filter &&
 		    DIFF_OPT_TST(&revs->diffopt, HAS_CHANGES))
 			break;
 

t/t4037-whitespace-status.sh

@@ -60,4 +60,11 @@ test_expect_success 'diff-files -b -p --exit-code' '
 	git diff-files -b -p --exit-code
 '
 
+test_expect_success 'diff-files --diff-filter --quiet' '
+	git reset --hard &&
+	rm a/d &&
+	echo x >>b/e &&
+	test_must_fail git diff-files --diff-filter=M --quiet
+'
+
 test_done