VC: ability to skip update buffers prompt

VC: ability to skip update buffers prompt

* lisp/vc/vc.el (vc-default-update-on-retrieve-tag): New function.
(vc-retrieve-tag): Call `update-on-retrieve-tag' backend function
to determine if prompt for update buffers is needed; Include tag
name into the "Retrieving tag" message.
* lisp/vc/vc-git.el (vc-git-update-on-retrieve-tag):
* lisp/vc/vc-hg.el (vc-hg-update-on-retrieve-tag):
* lisp/vc/vc-svn.el (vc-svn-udate-on-retrieve-tag): New functions.
Buffers update prompt on `vc-retrieve-tag' is omitted (bug#38156).