Convert memcpy(a,b,20) to hashcpy(a,b).

Convert memcpy(a,b,20) to hashcpy(a,b).

This abstracts away the size of the hash values when copying them
from memory location to memory location, much as the introduction
of hashcmp abstracted away hash value comparsion.

A few call sites were using char* rather than unsigned char* so
I added the cast rather than open hashcpy to be void*.  This is a
reasonable tradeoff as most call sites already use unsigned char*
and the existing hashcmp is also declared to be unsigned char*.

[jc: this is a follow-up patch for merge-recursive.c which is
 not in "master" yet.  The original was sent-in for "next" so
 I splitted it out. ]

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>