Improve syncing VC buffers before generating diffs

Improve syncing VC buffers before generating diffs

* lisp/vc/vc.el (vc-maybe-buffer-sync): Delete.  Correct
handling of indirect buffers is now implicitly achieved by
vc-buffer-sync-fileset.
(vc-buffer-sync-fileset): Make NOT-ESSENTIAL argument optional,
new MISSING-IN-DIRS optional argument.  Rewrite to handle
directories named in the fileset, not only files.
(vc-ediff): Replace call to vc-maybe-buffer-sync with a call to
vc-buffer-sync-fileset.
(vc-root-diff): Similarly replace call to vc-maybe-buffer-sync.
This means the user is prompted to save additional buffers, that
they likely want to save before generating the diffs.
* test/lisp/vc/vc-misc-tests.el: New file.

(cherry picked from commit 07c2b169edc2c5aaad1c8f494663a8198b2d4ca2)