Eliminate confusing "won't bisect on seeked tree" failure

Eliminate confusing "won't bisect on seeked tree" failure

This error message is very confusing---it doesn't tell the user
anything about how to fix the situation. And the actual fix
for the situation ("git bisect reset") does a checkout of a
potentially random branch, (compared to what the user wants to
be on for the bisect she is starting).

The simplest way to eliminate the confusion is to just make
"git bisect start" do the cleanup itself. There's no significant
loss of safety here since we already have a general safety in
the form of the reflog.

Note: We preserve the warning for any cogito users. We do this
by switching from .git/head-name to .git/BISECT_START for the
extra state, (which is a more descriptive name anyway).

Signed-off-by: Carl Worth <cworth@cworth.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>