bisect: check for mandatory argument of 'bisect replay'

bisect: check for mandatory argument of 'bisect replay'

'git bisect replay' has a mandatory logfile argument, but the current
implementation doesn't check whether the user has specified one.  When
the user omits the logfile argument, this leads to the following
unhelpful error message:

  cannot read  for replaying

So, check for the mandatory argument first, and provide a more
meaningful error message when it is omitted.

Signed-off-by: SZEDER Gábor <szeder@ira.uka.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>