mailsplit and mailinfo: gracefully handle NUL characters

The function fgets() has a big problem with NUL characters: it reads
them, but nobody will know if the NUL comes from the file stream, or
was appended at the end of the line.

So implement a custom read_line_with_nul() function.

Noticed by Tommy Thorn.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t5100-mailinfo.sh

@@ -25,4 +25,13 @@ do
 		diff ../t5100/info$mail info$mail"
 done
 
+test_expect_success 'respect NULs' '
+
+	git mailsplit -d3 -o. ../t5100/nul &&
+	cmp ../t5100/nul 001 &&
+	(cat 001 | git mailinfo msg patch) &&
+	test 4 = $(wc -l < patch)
+
+'
+
 test_done