patch 8.0.0177: BufEnter autocommand not fired for a directory

Problem:    When opening a buffer on a directory and inside a try/catch then
            the BufEnter event is not triggered.
Solution:   Return NOTDONE from readfile() for a directory and deal with the
            three possible return values. (Justin M. Keyes, closes #1375,
            closes #1353)

src/memline.c

@@ -1519,7 +1519,7 @@ ml_recover(void)
 			    line_count = pp->pb_pointer[idx].pe_line_count;
 			    if (readfile(curbuf->b_ffname, NULL, lnum,
 					pp->pb_pointer[idx].pe_old_lnum - 1,
-					line_count, NULL, 0) == FAIL)
+					line_count, NULL, 0) != OK)
 				cannot_open = TRUE;
 			    else
 				lnum += line_count;