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

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>