completion: wrap __git_refs() for better option parsing

completion: wrap __git_refs() for better option parsing

__git_refs() currently accepts two optional positional parameters: a
remote and a flag for 'git checkout's tracking DWIMery.  To fix a
minor bug, and, more importantly, for faster refs completion, this
series will add three more parameters: a prefix, the current word to
be completed and a suffix, i.e. the options accepted by __gitcomp() &
friends, and will change __git_refs() to list only refs matching that
given current word and to add that given prefix and suffix to the
listed refs.

However, __git_refs() is the helper function that is most likely used
in users' custom completion scriptlets for their own git commands, and
we don't want to break those, so

  - we can't change __git_refs()'s default output format, i.e. we
    can't by default append a trailing space to every listed ref,
    meaning that the suffix parameter containing the default trailing
    space would have to be specified on every invocation, and

  - we can't change the position of existing positional parameters
    either, so there would have to be plenty of set-but-empty
    placeholder positional parameters all over the completion script.

Furthermore, with five positional parameters it would be really hard
to remember which position means what.

To keep callsites simple, add the new wrapper function
__git_complete_refs() around __git_refs(), which:

  - instead of positional parameters accepts real '--opt=val'-style
    options and with minimalistic option parsing translates them to
    __git_refs()'s and __gitcomp_nl()'s positional parameters, and

  - includes the '__gitcomp_nl "$(__git_refs ...)" ...' command
    substitution to make its behavior match its name and the behavior
    of other __git_complete_* functions, and to limit future changes
    in this series to __git_refs() and this new wrapper function.

Call this wrapper function instead of __git_refs() wherever possible
throughout the completion script, i.e. when __git_refs()'s output is
fed to __gitcomp_nl() right away without further processing, which
means all callsites except a single one in the __git_refs2() helper.

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