checkout: forbid "-B <branch>" from touching a branch used elsewhere

"git checkout -B <branch> [<start-point>]", being a "forced" version
of "-b", switches to the <branch>, after optionally resetting its
tip to the <start-point>, even if the <branch> is in use in another
worktree, which is somewhat unexpected.

Protect the <branch> using the same logic that forbids "git checkout
<branch>" from touching a branch that is in use elsewhere.

This is a breaking change that may deserve backward compatibliity
warning in the Release Notes.  The "--ignore-other-worktrees" option
can be used as an escape hatch if the finger memory of existing
users depend on the current behaviour of "-B".

Reported-by: Willem Verstraeten <willem.verstraeten@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-checkout.txt

@@ -63,7 +63,9 @@ $ git checkout <branch>
 ------------
 +
 that is to say, the branch is not reset/created unless "git checkout" is
-successful.
+successful (e.g., when the branch is in use in another worktree, not
+just the current branch stays the same, but the branch is not reset to
+the start-point, either).
 
 'git checkout' --detach [<branch>]::
 'git checkout' [--detach] <commit>::

Documentation/git-switch.txt

@@ -59,13 +59,18 @@ out at most one of `A` and `B`, in which case it defaults to `HEAD`.
 -c <new-branch>::
 --create <new-branch>::
 	Create a new branch named `<new-branch>` starting at
-	`<start-point>` before switching to the branch. This is a
-	convenient shortcut for:
+	`<start-point>` before switching to the branch. This is the
+	transactional equivalent of
 +
 ------------
 $ git branch <new-branch>
 $ git switch <new-branch>
 ------------
++
+that is to say, the branch is not reset/created unless "git switch" is
+successful (e.g., when the branch is in use in another worktree, not
+just the current branch stays the same, but the branch is not reset to
+the start-point, either).
 
 -C <new-branch>::
 --force-create <new-branch>::