Improve handling of the first Git revision

Improve handling of the first Git revision

* lisp/vc/log-view.el (log-view-toggle-entry-display): When
there's no next entry, delete until the end of the buffer.
(log-view-end-of-defun-1): Stop at eob.

* lisp/vc/vc-annotate.el
(vc-annotate-show-diff-revision-at-line-internal): Don't give up
when previous-revision is nil.

* lisp/vc/vc-git.el (vc-git-expanded-log-entry): End the arguments
with `--' to avoid ambiguity.
(vc-git-annotate-extract-revision-at-line): Exclude `^' from the
returned revision string.
(vc-git-annotate-time): Expect `^' before the first revision.

* lisp/vc/vc-git.el (vc-git-diff): Diff against an empty tree if
REV1 is nil, and REV2 is not.

* lisp/vc/vc.el: Update the description of the `diff' function.