Noticed by Florian Forster: Use a char pointer when adding offsets,
because void pointer arithmetic is a GNU extension. Const'ify the
function arguments while we're at it.
Signed-off-by: Rene Scharfe <rene.scharfe@lsrfire.ath.cx> Signed-off-by: Junio C Hamano <junkio@cox.net>