diff: simplify run_external_diff() quiet logic

We'd sometimes end up in run_external_diff() to do a dry-run diff (e.g.,
to find content-level changes for --quiet). We recognize this quiet mode
by seeing the lack of DIFF_FORMAT_PATCH in the output format.

But since introducing an explicit dry-run check via 3ed5d8b (diff:
stop output garbled message in dry run mode, 2025-10-20), this logic can
never trigger. We can only get to this function by calling
diff_flush_patch(), and that comes from only two places:

  1. A dry-run flush comes from diff_flush_patch_quietly(), which is
     always in dry-run mode (so the other half of our "||" is true
     anyway).

  2. A regular flush comes from diff_flush_patch_all_file_pairs(),
     which is only called when output_format has DIFF_FORMAT_PATCH in
     it.

So we can simplify our "quiet" condition to just checking dry-run mode
(which used to be a specific flag, but recently became just a NULL
"file" pointer). And since it's so simple, we can just do that inline.
This makes the logic about o->file more obvious, since we handle the
NULL and non-stdout cases next to each other.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: peff

diff.c

@@ -4423,7 +4423,6 @@ static void run_external_diff(const struct external_diff *pgm,
 {
 	struct child_process cmd = CHILD_PROCESS_INIT;
 	struct diff_queue_struct *q = &diff_queued_diff;
-	int quiet = !(o->output_format & DIFF_FORMAT_PATCH) || !o->file;
 	int rc;
 
 	/*
@@ -4432,7 +4431,7 @@ static void run_external_diff(const struct external_diff *pgm,
 	 * external diff program lacks the ability to tell us whether
 	 * it's empty then we consider it non-empty without even asking.
 	 */
-	if (!pgm->trust_exit_code && quiet) {
+	if (!pgm->trust_exit_code && !o->file) {
 		o->found_changes = 1;
 		return;
 	}
@@ -4457,7 +4456,7 @@ static void run_external_diff(const struct external_diff *pgm,
 	diff_free_filespec_data(one);
 	diff_free_filespec_data(two);
 	cmd.use_shell = 1;
-	if (quiet)
+	if (!o->file)
 		cmd.no_stdout = 1;
 	else if (o->file != stdout)
 		cmd.out = xdup(fileno(o->file));