shada: Also store last search direction

Note: it looks like viminfo files do not store search direction intentionally.
After reading viminfo file search direction was considered to be “forward”.

Note 2: all files created on earlier Neovim version will automatically receive
“forward” direction.

Fixes #3580

src/nvim/search.c

@@ -4623,6 +4623,7 @@ void set_search_pattern(const SearchPattern pat)
 {
   free_spat(&spats[0]);
   memcpy(&(spats[0]), &pat, sizeof(spats[0]));
+  set_vv_searchforward();
 }
 
 /// Set last substitute pattern