patch 8.0.0334: can't access b:changedtick from a dict reference

Problem:    Can't access b:changedtick from a dict reference.
Solution:   Make changedtick a member of the b: dict. (inspired by neovim
            #6112)
diff --git a/src/fileio.c b/src/fileio.c
index 9380953..73baecc 100644
--- a/src/fileio.c
+++ b/src/fileio.c
@@ -4926,9 +4926,9 @@
 #ifdef FEAT_AUTOCMD
 	/* buf->b_changedtick is always incremented in unchanged() but that
 	 * should not trigger a TextChanged event. */
-	if (last_changedtick + 1 == buf->b_changedtick
+	if (last_changedtick + 1 == *buf->b_changedtick
 					       && last_changedtick_buf == buf)
-	    last_changedtick = buf->b_changedtick;
+	    last_changedtick = *buf->b_changedtick;
 #endif
 	u_unchanged(buf);
 	u_update_save_nr(buf);