Add Log View toggle and log format customization for Git and Hg.

Add Log View toggle and log format customization for Git and Hg.

* lisp/vc/vc-git.el (vc-git-root-log-format): New option for
customizing log format.
(vc-git-print-log, vc-git-log-outgoing, vc-git-log-incoming)
(vc-git-log-view-mode): Use it.
(vc-git-expanded-log-entry): New function.
(vc-git-log-view-mode): Use it.  Truncate lines in root log.

* lisp/vc/vc-hg.el (vc-hg-root-log-template): New option for
customizing log format.
(vc-hg-print-log): Use it.
(vc-hg-expanded-log-entry): New function.
(vc-hg-log-view-mode): Use vc-hg-root-log-template and
vc-hg-expanded-log-entry.  Truncate lines in root log.

* lisp/vc/vc-bzr.el (vc-bzr-log-view-mode): Truncate lines in root log.

* lisp/vc/log-view.el (log-view-mode-menu): Add
log-view-toggle-entry-display.