bash completion: complete CHERRY_PICK_HEAD

bash completion: complete CHERRY_PICK_HEAD

When e.g. in a failed cherry pick we did not recognize
CHERRY_PICK_HEAD as we do e.g. REBASE_HEAD in a failed rebase let's
rectify that.

When REBASE_HEAD was added in fbd7a232370 (rebase: introduce and use
pseudo-ref REBASE_HEAD, 2018-02-11) a completion was added for it, but
no corresponding completion existed for CHERRY_PICK_HEAD added in
d7e5c0cbfb0 (Introduce CHERRY_PICK_HEAD, 2011-02-19).

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>