Make revision completion in vc-diff and vc-root-diff more predictable

Make revision completion in vc-diff and vc-root-diff more predictable

* lisp/vc/vc.el (vc-root-version-diff):
Don't try calling 'vc-deduce-fileset', instead construct a fileset
suitable for the root directory right away.  This way the revision
completion for the root diff doesn't depend on the current buffer,
or the file at point (bug#73232).
(vc-diff-build-argument-list-internal): No special case when
invoked on a directory, or when the current file is not "dirty".
Make REV1-DEFAULT a string value.

* etc/NEWS: Mention the change.

(cherry picked from commit a295d7de9e8cae03fd4be6838d7eee59381791e4)