log-tree: always use return value of strbuf_detach()

strbuf_detach() has been returning a pointer to a buffer even for empty
strbufs since 08ad56f3f0 ("strbuf: always return a non-NULL value from
strbuf_detach", 2012-10-18).  Use that feature in show_log() instead of
having it handle empty strbufs specially.

Signed-off-by: René Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

log-tree.c

@@ -677,9 +677,7 @@ void show_log(struct rev_info *opt)
 		raw = (opt->commit_format == CMIT_FMT_USERFORMAT);
 		format_display_notes(&commit->object.oid, &notebuf,
 				     get_log_output_encoding(), raw);
-		ctx.notes_message = notebuf.len
+		ctx.notes_message = strbuf_detach(&notebuf, NULL);
-			? strbuf_detach(&notebuf, NULL)
-			: xcalloc(1, 1);
 	}
 
 	/*