t6300: test sort with multiple keys

Documentation of git-for-each-ref says that --sort=<key> option can be
used multiple times, in which case the last key becomes the primary key.
However this functionality was never checked in test suite and is
currently broken. This commit adds appropriate test in preparation for fix.

Signed-off-by: Kacper Kornet <draenog@pld-linux.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t6300-for-each-ref.sh

@@ -456,4 +456,14 @@ test_atom refs/tags/signed-long contents "subject line
 body contents
 $sig"
 
+cat >expected <<\EOF
+408fe76d02a785a006c2e9c669b7be5589ede96d <committer@example.com> refs/tags/master
+90b5ebede4899eda64893bc2a4c8f1d6fb6dfc40 <committer@example.com> refs/tags/bogo
+EOF
+
+test_expect_failure 'Verify sort with multiple keys' '
+	git for-each-ref --format="%(objectname) %(taggeremail) %(refname)" --sort=objectname --sort=taggeremail \
+		refs/tags/bogo refs/tags/master > actual &&
+	test_cmp expected actual
+'
 test_done