Bisect: add checks at the beginning of "git bisect run".

Bisect: add checks at the beginning of "git bisect run".

We may be able to "run" with only one good revision given
and then verify that the result of the first run is bad.
And perhaps also the other way around.

But for now let's check that we have at least one bad and
one good revision before we start to run.

Signed-off-by: Christian Couder <redacted>
Signed-off-by: Junio C Hamano <redacted>