status, branch: fix the misleading "bisecting" message

The current message is "bisecting %s" (or "bisecting branch %s").
"%s" is the current branch when we started bisecting. Clarify that to
avoid confusion with good and bad refs passed to "bisect" command.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>