format-patch: check that header line has expected format

The format of the "From " header line is very specific to allow
utilities to detect Git-style patches.  Add a test that the patches
created are in the expected format.
Signed-off-by: Nbrian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

t/t4014-format-patch.sh

@@ -1438,4 +1438,11 @@ test_expect_success 'format-patch --zero-commit' '
 	test_cmp expect actual
 '
 
+test_expect_success 'From line has expected format' '
+	git format-patch --stdout v2..v1 >patch2 &&
+	grep "^From " patch2 >from &&
+	grep "^From $_x40 Mon Sep 17 00:00:00 2001$" patch2 >filtered &&
+	test_cmp from filtered
+'
+
 test_done