patch 8.1.1374: check for file changed triggers too often

Problem:    Check for file changed triggers too often.
Solution:   Don't use "b_p_ar" when it is negative.

src/fileio.c

@@ -6776,7 +6776,7 @@ buf_check_timestamp(
 	{
 	    // 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_mtime = (buf->b_p_ar >= 0 ? buf->b_p_ar : p_ar);
 	    buf->b_orig_size = 0;
 	    buf->b_orig_mode = 0;
 	}

src/version.c

@@ -767,6 +767,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1374,
 /**/
     1373,
 /**/