git-submodule summary: --for-status option

The --for-status option is mainly used by builtin-status/commit.
It adds 'Modified submodules:' line at top and  '# ' prefix to all
following lines.

Signed-off-by: Ping Yin <pkufranky@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t7401-submodule-summary.sh

@@ -192,4 +192,17 @@ test_expect_success 'given commit' "
 EOF
 "
 
+test_expect_success '--for-status' "
+    git submodule summary --for-status HEAD^ >actual &&
+    test_cmp actual - <<EOF
+# Modified submodules:
+#
+# * sm1 $head6...0000000:
+#
+# * sm2 0000000...$head7 (2):
+#   > Add foo9
+#
+EOF
+"
+
 test_done