run_external_diff: clean up error handling

When the external diff reports an error, we try to clean up
and die. However, we can make this process a bit simpler:

  1. We do not need to bother freeing memory, since we are
     about to exit.  Nor do we need to clean up our
     tempfiles, since the atexit() handler will do it for
     us. So we can die as soon as we see the error.

  3. We can just call die() rather than fprintf/exit. This
     does technically change our exit code, but the exit
     code of "1" is not meaningful here. In fact, it is
     probably wrong, since "1" from diff usually means
     "completed successfully, but there were differences".

And while we're there, we can mark the error message for
translation, and drop the full stop at the end to make it
more like our other messages.

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

diff.c

@@ -2909,7 +2909,6 @@ static void run_external_diff(const char *pgm,
 {
 	struct argv_array argv = ARGV_ARRAY_INIT;
 	struct argv_array env = ARGV_ARRAY_INIT;
-	int retval;
 	struct diff_queue_struct *q = &diff_queued_diff;
 
 	if (one && two) {
@@ -2938,14 +2937,12 @@ static void run_external_diff(const char *pgm,
 	argv_array_pushf(&env, "GIT_DIFF_PATH_COUNTER=%d", ++o->diff_path_counter);
 	argv_array_pushf(&env, "GIT_DIFF_PATH_TOTAL=%d", q->nr);
 
-	retval = run_command_v_opt_cd_env(argv.argv, RUN_USING_SHELL, NULL, env.argv);
+	if (run_command_v_opt_cd_env(argv.argv, RUN_USING_SHELL, NULL, env.argv))
+		die(_("external diff died, stopping at %s"), name);
+
 	remove_tempfile();
 	argv_array_clear(&argv);
 	argv_array_clear(&env);
-	if (retval) {
-		fprintf(stderr, "external diff died, stopping at %s.\n", name);
-		exit(1);
-	}
 }
 
 static int similarity_index(struct diff_filepair *p)

t/t7800-difftool.sh

@@ -58,7 +58,7 @@ test_expect_success PERL 'custom tool commands override built-ins' '
 
 test_expect_success PERL 'difftool ignores bad --tool values' '
 	: >expect &&
-	test_expect_code 1 \
+	test_must_fail \
 		git difftool --no-prompt --tool=bad-tool branch >actual &&
 	test_cmp expect actual
 '