vim-patch:8.2.1741: pathshorten() only supports using one character

Problem:    pathshorten() only supports using one character.
Solution:   Add an argument to control the length. (closes vim/vim#7006)
6a33ef0deb

Cherry-pick a line in test from patch 8.2.0634.
Use Nvim's config paths in docs.

shorten_dir() returning a pointer looks a bit confusing here, as it is
actually the same pointer passed to it, and it doesn't really reduce
much code, so change it back to void.

Assigning rettv->vval.v_string = NULL is not needed if a pointer is
within 64 bits. While this is usually the case, I'm not sure if it can
be taken for granted.

src/nvim/screen.c

@@ -7350,7 +7350,7 @@ void draw_tabline(void)
       if (room > 0) {
         // Get buffer name in NameBuff[]
         get_trans_bufname(cwp->w_buffer);
-        (void)shorten_dir(NameBuff);
+        shorten_dir(NameBuff);
         len = vim_strsize(NameBuff);
         p = NameBuff;
         while (len > room) {