patch 8.1.0161: buffer not updated with 'autoread' set if file was deleted

Problem:    Buffer not updated with 'autoread' set if file was deleted.
            (Michael Naumann)
Solution:   Don't set the timestamp to zero. (closes #3165)
diff --git a/src/fileio.c b/src/fileio.c
index 0c56a1e..07b241e 100644
--- a/src/fileio.c
+++ b/src/fileio.c
@@ -6923,11 +6923,13 @@
     {
 	retval = 1;
 
-	/* set b_mtime to stop further warnings (e.g., when executing
-	 * FileChangedShell autocmd) */
+	// set b_mtime to stop further warnings (e.g., when executing
+	// FileChangedShell autocmd)
 	if (stat_res < 0)
 	{
-	    buf->b_mtime = 0;
+	    // When 'autoread' is set we'll check the file again to see if it
+	    // re-appears.
+	    buf->b_mtime = buf->b_p_ar;
 	    buf->b_orig_size = 0;
 	    buf->b_orig_mode = 0;
 	}