Support 'vc-region-history' for Mercurial

Support 'vc-region-history' for Mercurial

* lisp/vc/vc-hg.el (vc-hg-region-history)
(vc-hg-region-history-font-lock, vc-hg-region-history-mode):
New functions.
(vc-hg-region-history-mode-map)
(vc-hg--log-view-long-font-lock-keywords)
(vc-hg-region-history-font-lock-keywords): New variables.
* lisp/vc/vc-git.el (vc-git-region-history): Update commentary.

* doc/emacs/maintaining.texi (VC Change Log): Add
'vc-region-history' to the table at beginning of node.  Update
the VCSes that support 'vc-region-history'.

* etc/NEWS: Mention the new feature of vc-hg.el.