completion: support completing full refs after '--option=refs/<TAB>'

completion: support completing full refs after '--option=refs/<TAB>'

Completing full refs currently only works when the full ref stands on
in its own on the command line, but doesn't work when the current word
to be completed contains a prefix before the full ref, e.g.
'--option=refs/<TAB>' or 'master..refs/bis<TAB>'.

The reason is that __git_refs() looks at the current word to be
completed ($cur) as a whole to decide whether it has to list full (if
it starts with 'refs/') or short refs (otherwise).  However, $cur also
holds said '--option=' or 'master..' prefixes, which of course throw
off this decision.  Luckily, the default action is to list short refs,
that's why completing short refs happens to work even after a
'master..<TAB>' prefix and similar cases.

Pass only the ref part of the current word to be completed to
__git_refs() as a new positional parameter, so it can make the right
decision even if the whole current word contains some kind of a
prefix.

Make this new parameter the 4. positional parameter and leave the 3.
as an ignored placeholder for now (it will be used later in this patch
series).

Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>