Support showing a single log entry from vc-annotate.

Support showing a single log entry from vc-annotate.
* vc.el (print-log): Add a new argument: START-REVISION.
(vc-print-log-internal): Add a new optional argument and
pass it to the backend.
(vc-print-log, vc-print-root-log): Adjust callers.
* vc-annotate.el (vc-annotate-show-log-revision-at-line): If a
buffer already displays the requested log entry, use it.
Otherwise display only the log entry in question.
* vc-svn.el (vc-svn-print-log):
* vc-mtn.el (log-view-file-re):
* vc-hg.el (vc-hg-state):
* vc-git.el (vc-git-print-log): Add support for new argument START-REVISION.
(vc-git-show-log-entry): Return t on success.
* vc-bzr.el (vc-bzr-print-log): Add support new argument START-REVISION.
(vc-bzr-show-log-entry): Return t on success.
* vc-rcs.el (vc-rcs-print-log):
* vc-sccs.el (vc-sccs-print-log):
* vc-cvs.el (vc-cvs-print-log): Add new argument, ignore it.