Add a bit more information to "git status" during a rebase/bisect
session.

* nd/branch-show-rebase-bisect-state:
  status, branch: fix the misleading "bisecting" message
  branch: show more information when HEAD is detached
  status: show more info than "currently not on any branch"
  wt-status: move wt_status_get_state() out to wt_status_print()
  wt-status: split wt_status_state parsing function out
  wt-status: move strbuf into read_and_strip_branch()