Un-obsolete dired-in-this-tree-p and use it again in certain places.

Un-obsolete dired-in-this-tree-p and use it again in certain places.

This reverts parts of b425966b07, and 7b50ed553f, i.e. it reverts the
obsoletion of dired-in-this-tree-p and switches some new callers of
file-in-directory-p back to using dired-in-this-tree-p.

It turned out that using file-in-directory-p can be a major
performance regression in case you have a dired buffer of a remote
directory which has become (almost) inaccessible.

Any attempt to open a new dired buffer is checking if a buffer for
that directory already exists (in terms of dired-buffers-for-dir)
which meant that file-in-directory-p was called with the directory of
any existing dired buffer including the inaccessible one where the
file-truename call in file-in-directory-p could block for seconds or
even minutes.

* lisp/dired.el (dired-in-this-tree-p): Undo obsoletion.
(dired-buffers-for-dir): Use dired-in-this-tree-p as before.
* lisp/dired-aux.el (dired-kill-tree,dired-tree-down): Une
dired-in-this-tree-p as before.