When rewriting, flush the new log earlier

... such that LRU, which is operating on the temporary log, can
make room.

Ref #28

src/fellow_log.c

@@ -3317,6 +3317,8 @@ logbuffer_append(struct fellow_fd *ffd,
 	AN(off);
 
 	if (locked == 0) {
+		// flush destination to get disk blocks #28
+		logbuffer_flush(ffd, to, 0, LBUF_ALL);
 		/* avoid chasing the tail forever
 		 * XXX good threshold?
 		 */