vc-start-logentry: Use current buffer as parent buffer more often

vc-start-logentry: Use current buffer as parent buffer more often

* lisp/vc/vc-dispatcher.el (vc-start-logentry): When determining
the parent buffer, if get-file-buffer returns nil, use the
current buffer.  Also, if the first file is a directory, don't
try to call get-file-buffer, just use the current buffer.

(cherry picked from commit 475a33adb9134990fa05be8a6308216f12ed4ef7)