vc-checkin: Check whether the fileset or patches have changed

vc-checkin: Check whether the fileset or patches have changed

* lisp/vc/vc-dispatcher.el (vc-finish-logentry): Delay popping
to vc-parent-buffer until after calling the log operation.
That way if the log operation exits early, the current buffer
remains *vc-log*.
(vc-dir-marked-files, dired-get-marked-files): Declare.
(vc-dispatcher--explicit-marks-p): New function.
* lisp/vc/vc.el (vc-checkin): Check the user isn't likely to be
surprised by what is included in the checkin.  Specifically,
check whether the fileset or patches implied by vc-parent-buffer
are unchanged.
* doc/emacs/maintaining.texi (VC With A Merging VCS): Explain
how the fileset or patch string is fixed once *vc-log* pops up.

(cherry picked from commit 7a0bfa3ee7fa447a0fe994ac8f64bcb5752cacb2)