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>
diff --git a/t/t6300-for-each-ref.sh b/t/t6300-for-each-ref.sh
index 1721784..a0d82d4 100755
--- a/t/t6300-for-each-ref.sh
+++ b/t/t6300-for-each-ref.sh
@@ -456,4 +456,14 @@
 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