pretty.c: add %x00 format specifier.

This adds a %xXX format which inserts two hexdigits after %x as a byte
value in the resulting string.  This can be used to add a NUL byte or any
other byte that can make machine parsing easier.  It is also necessary to
use fwrite to print out the data since printf will terminate if you feed
it a NUL.

Signed-off-by: Govind Salinas <blix@sophiasuchtig.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/pretty-formats.txt

@@ -123,3 +123,4 @@ The placeholders are:
 - '%Creset': reset color
 - '%m': left, right or boundary mark
 - '%n': newline
+- '%x00': print a byte from a hex code