patch 9.1.0231: Filetype may be undetected when SwapExists sets ft in other buf

Problem:  Filetype may be undetected when a SwapExists autocommand sets
          filetype in another buffer.
Solution: Make filetype detection state buffer-specific.  Also fix a
          similar problem for 'modified' (zeertzjq).

closes: #14344

Signed-off-by: zeertzjq <zeertzjq@outlook.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/src/ex_cmds.c b/src/ex_cmds.c
index 864f89d..2a5d842 100644
--- a/src/ex_cmds.c
+++ b/src/ex_cmds.c
@@ -2961,7 +2961,7 @@
     // Since we are starting to edit a file, consider the filetype to be
     // unset.  Helps for when an autocommand changes files and expects syntax
     // highlighting to work in the other file.
-    did_filetype = FALSE;
+    curbuf->b_did_filetype = FALSE;
 
 /*
  * other_file	oldbuf