grep: use return value of strbuf_detach()

Append the strbuf buffer only after detaching it.  There is no practical
difference here, as the strbuf is not empty and no strbuf_ function is
called between storing the pointer to the still attached buffer and
calling strbuf_detach(), so that pointer is valid, but make sure to
follow the standard sequence anyway for consistency.

Signed-off-by: René Scharfe <l.s.r@web.de>
Acked-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/grep.c

@@ -1110,8 +1110,8 @@ int cmd_grep(int argc, const char **argv, const char *prefix)
 			strbuf_addf(&buf, "+/%s%s",
 					strcmp("less", pager) ? "" : "*",
 					opt.pattern_list->pattern);
-			string_list_append(&path_list, buf.buf);
-			strbuf_detach(&buf, NULL);
+			string_list_append(&path_list,
+					   strbuf_detach(&buf, NULL));
 		}
 	}