Verify that stderr is taken into account

test/git-check-assertions.bats

@@ -170,3 +170,12 @@ commit_with_assertion() {
   assert_output --partial "Expected output to equal: goodbye"
   assert_output --partial "Actual output: hello"
 }
+
+@test "assert_output should also check stderr output" {
+  git checkout -b feature
+  commit_with_assertion $'run sh -c "echo err 1>&2"\nassert_output err'
+
+  run git-check-assertions
+
+  assert_success
+}