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

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

In October I installed a change to vc-diff-patch-string and
labelled it as "Fix window selection after log-edit-show-diff"
but that change only fixed the problem within
log-view-modify-change-comment.  This change should fix window
selection after C-c C-d in ordinary checkins.

* lisp/vc/log-edit.el (log-edit-diff-fileset): Use
save-selected-window in order to respect the documented
requirement that values of log-edit-diff-function leave the Log
Edit buffer's window selected when they return.

(cherry picked from commit d283db577339ee247826bfa1e8c1808dad823858)