bisect: allow setting any user-specified in 'git bisect start'

This allows a natural user-interface when looking for any change in the
code, not just regression. For example:

git bisect start --term-old fast --term-new slow
git bisect fast
git bisect slow
...

There were several proposed user-interfaces for this feature. This patch
implements it as options to 'git bisect start' for the following reasons:

* By construction, the terms will be valid for one and only one
  bisection.

* Unlike positional arguments, using named options avoid having to
  remember an order.

* We can combine user-defined terms and passing old/new commits as
  argument to "git bisect start".

* The implementation is relatively simple.

See previous discussions:

  http://mid.gmane.org/1435337896-20709-3-git-send-email-Matthieu.Moy@imag.fr

Signed-off-by: Matthieu Moy <Matthieu.Moy@imag.fr>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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