completion: bisect: complete custom terms and related options

completion: bisect: complete custom terms and related options

git bisect supports the use of custom terms via the --term-(new|bad) and
--term-(old|good) options, but the completion code doesn't know about
these options or the new subcommands they define.

Add support for these options and the custom subcommands by checking for
BISECT_TERMS and adding them to the list of subcommands.  Add tests.

Signed-off-by: Britton Leo Kerin <britton.kerin@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>