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/option.c b/src/option.c
index 8123a2a..c594301 100644
--- a/src/option.c
+++ b/src/option.c
@@ -3678,7 +3678,7 @@
     if (!args->os_newval.boolean)
 	save_file_ff(curbuf);	// Buffer is unchanged
     redraw_titles();
-    modified_was_set = args->os_newval.boolean;
+    curbuf->b_modified_was_set = args->os_newval.boolean;
     return NULL;
 }