fix overlapping memcpy in normalize_absolute_path

fix overlapping memcpy in normalize_absolute_path

The comments for normalize_absolute_path explicitly claim
that the source and destination buffers may be the same
(though they may not otherwise overlap). Thus the call to
memcpy may involve copying overlapping data, and memmove
should be used instead.

This fixes a valgrind error in t1504.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>