patch 8.0.1633: a TextChanged autocmd triggers when it is defined

Problem:    A TextChanged autocmd triggers when it is defined after creating a
            buffer.
Solution:   Set b_last_changedtick when opening a buffer. (Hirohito Highlight,
            closes #2742)
diff --git a/src/buffer.c b/src/buffer.c
index b292150..c7f361a 100644
--- a/src/buffer.c
+++ b/src/buffer.c
@@ -291,6 +291,13 @@
 	unchanged(curbuf, FALSE);
     save_file_ff(curbuf);		/* keep this fileformat */
 
+    /* Set last_changedtick to avoid triggering a TextChanged autocommand right
+     * after it was added. */
+    curbuf->b_last_changedtick = CHANGEDTICK(curbuf);
+#ifdef FEAT_INS_EXPAND
+    curbuf->b_last_changedtick_pum = CHANGEDTICK(curbuf);
+#endif
+
     /* require "!" to overwrite the file, because it wasn't read completely */
 #ifdef FEAT_EVAL
     if (aborting())