bisect--helper: use BISECT_TERMS in 'bisect skip' command

bisect--helper: use BISECT_TERMS in 'bisect skip' command

Commit e4c7b33747 ("bisect--helper: reimplement `bisect_skip` shell
function in C", 2021-02-03), as part of the shell-to-C conversion,
forgot to read the 'terms' file (.git/BISECT_TERMS) during the new
'bisect skip' command implementation. As a result, the 'bisect skip'
command will use the default 'bad'/'good' terms. If the bisection
terms have been set to non-default values (for example by the
'bisect start' command), then the 'bisect skip' command will fail.

In order to correct this problem, we insert a call to the get_terms()
function, which reads the non-default terms from that file (if set),
in the '--bisect-skip' command implementation of 'bisect--helper'.

Also, add a test[1] to protect against potential future regression.

[1] https://lore.kernel.org/git/xmqqim45h585.fsf@gitster.g/T/#m207791568054b0f8cf1a3942878ea36293273c7d

Reported-by: Trygve Aaberge <trygveaa@gmail.com>
Signed-off-by: Bagas Sanjaya <bagasdotme@gmail.com>
Signed-off-by: Ramsay Jones <ramsay@ramsayjones.plus.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>