completion: merge options for cherry-pick and revert

completion: merge options for cherry-pick and revert

Since revert and cherry-pick share the same sequencer code, they should
both accept the same command-line options. Derive the
`__git_cherry_pick_inprogress_options` and
`__git_revert_inprogress_options` variables from
`__git_sequencer_inprogress_options` so that the options aren't
unnecessarily duplicated twice.

Signed-off-by: Denton Liu <liu.denton@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>