send-email: fix missing error message regression

Fix a regression with the "the editor exited uncleanly, aborting
everything" error message going missing after my
d21616c (git-send-email: refactor duplicate $? checks into a
function, 2021-04-06).

I introduced a $msg variable, but did not actually use it. This caused
us to miss the optional error message supplied by the "do_edit"
codepath. Fix that, and add tests to check that this works.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: avar

git-send-email.perl

@@ -219,8 +219,18 @@ sub system_or_msg {
 	my $exit_code = $? >> 8;
 	return unless $signalled or $exit_code;
 
+	my @sprintf_args = ($args->[0], $exit_code);
+	if (defined $msg) {
+		# Quiet the 'redundant' warning category, except we
+		# need to support down to Perl 5.8, so we can't do a
+		# "no warnings 'redundant'", since that category was
+		# introduced in perl 5.22, and asking for it will die
+		# on older perls.
+		no warnings;
+		return sprintf($msg, @sprintf_args);
+	}
 	return sprintf(__("fatal: command '%s' died with exit code %d"),
-		       $args->[0], $exit_code);
+		       @sprintf_args);
 }
 
 sub system_or_die {

t/t9001-send-email.sh

@@ -644,14 +644,33 @@ test_expect_success $PREREQ 'In-Reply-To with --chain-reply-to' '
 	test_cmp expect actual
 '
 
+test_set_editor "$(pwd)/fake-editor"
+
+test_expect_success $PREREQ 'setup erroring fake editor' '
+	write_script fake-editor <<-\EOF
+	echo >&2 "I am about to error"
+	exit 1
+	EOF
+'
+
+test_expect_success $PREREQ 'fake editor dies with error' '
+	clean_fake_sendmail &&
+	test_must_fail git send-email \
+		--compose --subject foo \
+		--from="Example <nobody@example.com>" \
+		--to=nobody@example.com \
+		--smtp-server="$(pwd)/fake.sendmail" \
+		$patches 2>err &&
+	grep "I am about to error" err &&
+	grep "the editor exited uncleanly, aborting everything" err
+'
+
 test_expect_success $PREREQ 'setup fake editor' '
 	write_script fake-editor <<-\EOF
 	echo fake edit >>"$1"
 	EOF
 '
 
-test_set_editor "$(pwd)/fake-editor"
-
 test_expect_success $PREREQ '--compose works' '
 	clean_fake_sendmail &&
 	git send-email \