Improved ChangeLog generation for vc log (Bug#16301)

Improved ChangeLog generation for vc log (Bug#16301)

* lisp/vc/diff-mode.el (diff-find-source-location): Fix docstring.

* lisp/vc/add-log.el (change-log-unindented-file-names-re)
(change-log-read-entries, change-log-read-defuns)
(change-log-insert-entries):
* lisp/vc/diff-mode.el (diff-add-log-current-defuns):
* lisp/vc/log-edit.el (log-edit--insert-filled-defuns)
(log-edit-fill-entry): New functions.
(log-edit-mode): Set `log-edit-fill-entry' as
`fill-paragraph-function'.
(log-edit-generate-changelog-from-diff): New command.
(log-edit-mode-map): Bind it to C-c C-w.
* doc/emacs/maintaining.texi (Types of Log File, Log Buffer):
* CONTRIBUTE: Document it.
* etc/NEWS: Announce it.
* test/lisp/vc/log-edit-tests.el (log-edit-fill-entry)
(log-edit-fill-entry-joining): New tests.