pretty: use prefixcmp instead of memcmp on NUL-terminated strings

This conversion avoids the need for magic string length numbers in the
code.  And unlike memcmp(), prefixcmp() is careful to not run over the
end of a string.

Signed-off-by: Rene Scharfe <rene.scharfe@lsrfire.ath.cx>
Signed-off-by: Junio C Hamano <gitster@pobox.com>