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>

csum-file.h

@@ -62,4 +62,11 @@ static inline void hashwrite_be32(struct hashfile *f, uint32_t data)
 	hashwrite(f, &data, sizeof(data));
 }
 
+static inline size_t hashwrite_be64(struct hashfile *f, uint64_t data)
+{
+	data = htonll(data);
+	hashwrite(f, &data, sizeof(data));
+	return sizeof(data);
+}
+
 #endif