'C-x v b' prefix key is used for branch commands to create/switch/print branch

'C-x v b' prefix key is used for branch commands to create/switch/print branch

* lisp/vc/vc.el (vc-create-branch): New command.
(vc-retrieve-tag): Add new optional arg 'branchp'.
(vc-switch-branch): New command (bug#50344).

* lisp/vc/vc-hooks.el (vc-prefix-map): Bind "b c" to vc-create-branch,
"b l" to vc-print-branch-log, "b s" to vc-switch-branch.  Remove obsolete
and suppressed "b" from vc-switch-backend.

* lisp/vc/vc-dir.el (vc-dir-mode-map): Rebind 'branch-map' from "B" to "b"
for consistency with 'vc-prefix-map'.

* lisp/vc/vc-git.el (vc-git-create-tag): For a new branch read
its start-point.  Ask a confirmation if modified files exist.