branch: report active bisect run when rejecting delete

branch: report active bisect run when rejecting delete

git branch refuses to delete branches that are currently checked out
with a message like this: "error: cannot delete branch 'foo' used by
worktree at '/path/of/worktree'".  This can be confusing if it's an
internal checkout for git bisect.  Report a more specific error in
that case to help users that might have forgotten their bisect run.

Suggested-by: stsp <stsp2@yandex.ru>
Signed-off-by: René Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>