emacs: Use pop-to-buffer-same-window rather than switch-to-buffer

emacs: Use pop-to-buffer-same-window rather than switch-to-buffer

This means that notmuch commands obey display-buffer-alist so the user
can customize how buffers show up.

It also permits the use of C-x 4 4, C-x 5 5 and C-x t t, available in
Emacs 28.  For example, one can use C-x 4 4 M-x notmuch-jump-search RET
to open a saved search in another window rather than the current window.
Or in notmuch-search mode, C-x 5 5 RET to view the message at point in
a new frame.

notmuch-tree has custom buffer display logic, so bind
display-buffer-overriding-action to make pop-to-buffer-same-window
behave exactly as switch-to-buffer while that function is running.