log-edit-diff-function leaves the diff-mode buffer's window selected

log-edit-diff-function leaves the diff-mode buffer's window selected

This reverts the following two changesets:

    Author:     Sean Whitton <spwhitton@spwhitton.name>
    AuthorDate: Tue Oct 29 09:40:02 2024 +0800

      Fix window selection after log-edit-show-diff

    Author:     Sean Whitton <spwhitton@spwhitton.name>
    AuthorDate: Tue Apr 8 20:36:42 2025 +0800

      Fix window selection after log-edit-show-diff, again

This also replaces the following changeset:

    Author:     Sean Whitton <spwhitton@spwhitton.name>
    AuthorDate: Tue Oct 29 12:52:20 2024 +0800

      * lisp/vc/log-edit.el (log-edit-diff-function): Document.

* lisp/vc/log-edit.el (log-edit-diff-function): Document that
the function *should* leave the window selected when it returns.
(log-edit-diff-fileset):
* lisp/vc/vc.el (vc-modify-change-comment):
Don't use save-selected-window (bug#77946).
(vc-diff-patch-string):
Call pop-to-buffer, not display-buffer (bug#77946).

(cherry picked from commit d44c87ceb6dc7e85ccfecf47f52f8495e34c1d0b)