Add a testcase that exercises the logic when an invalid output format is
passed via the `fetch.output` configuration.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
---
 t/t5574-fetch-output.sh | 17 +++++++++++++++++
 1 file changed, 17 insertions(+)

diff --git a/t/t5574-fetch-output.sh b/t/t5574-fetch-output.sh
index f91b654d38..0e45c27007 100755
--- a/t/t5574-fetch-output.sh
+++ b/t/t5574-fetch-output.sh
@@ -7,6 +7,23 @@ export GIT_TEST_DEFAULT_INITIAL_BRANCH_NAME
 
 . ./test-lib.sh
 
+test_expect_success 'fetch with invalid output format configuration' '
+	test_when_finished "rm -rf clone" &&
+	git clone . clone &&
+
+	test_must_fail git -C clone -c fetch.output= fetch origin >actual 2>&1 &&
+	cat >expect <<-EOF &&
+	fatal: invalid value for ${SQ}fetch.output${SQ}: ${SQ}${SQ}
+	EOF
+	test_cmp expect actual &&
+
+	test_must_fail git -C clone -c fetch.output=garbage fetch origin >actual 2>&1 &&
+	cat >expect <<-EOF &&
+	fatal: invalid value for ${SQ}fetch.output${SQ}: ${SQ}garbage${SQ}
+	EOF
+	test_cmp expect actual
+'
+
 test_expect_success 'fetch aligned output' '
 	git clone . full-output &&
 	test_commit looooooooooooong-tag &&
-- 
2.40.0