unpack: replace xwrite() loop with write_in_full()

We have two packfile stream consumers, index-pack and
unpack-objects, that allow excess payload after the packfile stream
data. Their code to relay excess data hasn't changed significantly
since their original implementation that appeared in 67e5a5ec
(git-unpack-objects: re-write to read from stdin, 2005-06-28) and
9bee2478 (mimic unpack-objects when --stdin is used with index-pack,
2006-10-25).

These code blocks contain hand-rolled loops using xwrite(), written
before our write_in_full() helper existed. This helper now provides
the same functionality.

Replace these loops with write_in_full() for shorter, clearer
code. Update related variables accordingly.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/unpack-objects.c

@@ -679,13 +679,7 @@ int cmd_unpack_objects(int argc, const char **argv, const char *prefix UNUSED)
 	use(the_hash_algo->rawsz);
 
 	/* Write the last part of the buffer to stdout */
-	while (len) {
-		int ret = xwrite(1, buffer + offset, len);
-		if (ret <= 0)
-			break;
-		len -= ret;
-		offset += ret;
-	}
+	write_in_full(1, buffer + offset, len);
 
 	/* All done */
 	return has_errors;