ref-filter: make "%(symref)" atom work with the ':short' modifier

The "%(symref)" atom doesn't work when used with the ':short' modifier
because we strictly match only 'symref' for setting the 'need_symref'
indicator. Fix this by comparing with the valid_atom rather than the
used_atom.

Add tests for %(symref) and %(symref:short) while we're here.

Helped-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Karthik Nayak <Karthik.188@gmail.com>
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 af76dc5..7663a36 100755
--- a/t/t6300-for-each-ref.sh
+++ b/t/t6300-for-each-ref.sh
@@ -38,6 +38,7 @@
 	case "$1" in
 		head) ref=refs/heads/master ;;
 		 tag) ref=refs/tags/testtag ;;
+		 sym) ref=refs/heads/sym ;;
 		   *) ref=$1 ;;
 	esac
 	printf '%s\n' "$3" >expected
@@ -566,6 +567,7 @@
 	test_cmp expected actual
 '
 
+
 test_expect_success 'do not dereference NULL upon %(HEAD) on unborn branch' '
 	test_when_finished "git checkout master" &&
 	git for-each-ref --format="%(HEAD) %(refname:short)" refs/heads/ >actual &&
@@ -600,4 +602,26 @@
 	test_cmp expect actual.clean
 '
 
+test_expect_success 'Add symbolic ref for the following tests' '
+	git symbolic-ref refs/heads/sym refs/heads/master
+'
+
+cat >expected <<EOF
+refs/heads/master
+EOF
+
+test_expect_success 'Verify usage of %(symref) atom' '
+	git for-each-ref --format="%(symref)" refs/heads/sym >actual &&
+	test_cmp expected actual
+'
+
+cat >expected <<EOF
+heads/master
+EOF
+
+test_expect_success 'Verify usage of %(symref:short) atom' '
+	git for-each-ref --format="%(symref:short)" refs/heads/sym >actual &&
+	test_cmp expected actual
+'
+
 test_done