csum-file: add hashwrite_be64()

csum-file: add hashwrite_be64()

Add a helper function for hashing and writing 64-bit integers in network
byte order.  It returns the number of written bytes.  This simplifies
callers that keep track of the file offset, even though this number is a
constant.

Suggested-by: Derrick Stolee <dstolee@microsoft.com>
Original-patch-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: René Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>