pack-objects: fix comment of reused_chunk.difference

As record_reused_object(offset, offset - hashfile_total(out)) said,
reused_chunk.difference should be the offset of original packfile minus
the offset of the generated packfile. But the comment presented an opposite way.

Signed-off-by: Han Xin <hanxin.hx@alibaba-inc.com>
Acked-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/pack-objects.c

@@ -815,8 +815,8 @@ static struct reused_chunk {
 	/* The offset of the first object of this chunk in the original
 	 * packfile. */
 	off_t original;
-	/* The offset of the first object of this chunk in the generated
-	 * packfile minus "original". */
+	/* The difference for "original" minus the offset of the first object of
+	 * this chunk in the generated packfile. */
 	off_t difference;
 } *reused_chunks;
 static int reused_chunks_nr;