New option to make 'C-x 4 a' use file-less ChangeLog buffers

New option to make 'C-x 4 a' use file-less ChangeLog buffers

* doc/emacs/maintaining.texi (Change Log Commands): Document
add-log-dont-create-changelog-file.

* etc/NEWS (Change Logs Mode): Mention
add-log-dont-create-changelog-file.

* lisp/vc/add-log.el (add-log-file-name): Add comment.
(add-log-dont-create-changelog-file): New variable.
(add-log--pseudo-changelog-buffer-name)
(add-log--changelog-buffer-p): New helpers.
(add-log-find-changelog-buffer): New function.
(add-log--pseudo-changelog-buffer-name): Respect
add-log-dont-create-changelog-file.

* lisp/vc/log-edit.el (log-edit-changelog-entries): Use
add-log-find-changelog-buffer.