prefer memcpy to strcpy

prefer memcpy to strcpy

When we already know the length of a string (e.g., because
we just malloc'd to fit it), it's nicer to use memcpy than
strcpy, as it makes it more obvious that we are not going to
overflow the buffer (because the size we pass matches the
size in the allocation).

This also eliminates calls to strcpy, which make auditing
the code base harder.

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