completion: let 'for-each-ref' and 'ls-remote' filter matching refs

When completing refs, several __git_refs() code paths list all the
refs from the refs/{heads,tags,remotes}/ hierarchy and then
__gitcomp_nl() iterates over those refs in a shell loop to filter out
refs not matching the current ref to be completed.  This comes with a
considerable performance penalty when a repository contains a lot of
refs but the current ref can be uniquely completed or when only a
handful of refs match the current ref.

Reduce the number of iterations in __gitcomp_nl() from the number of
refs to the number of matching refs by specifying appropriate globbing
patterns to 'git for-each-ref' and 'git ls-remote' to list only those
refs that match the current ref to be completed.  However, do so only
when the ref to match is explicitly given as parameter, because the
current word on the command line might contain a prefix like
'--option=' or 'branch..'.  The __git_complete_refs() and
__git_complete_fetch_refspecs() helpers introduced previously in this
patch series already call __git_refs() specifying this current ref
parameter, so all their callsites, i.e. all places in the completion
script doing refs completion, can benefit from this optimization.

Furthermore, list only those symbolic and pseudo refs that match the
current ref to be completed.  Though it doesn't matter at all in
itself performance-wise, it will allow us further significant
optimizations later in this series.

This speeds up refs completion considerably when there are a lot of
non-matching refs to be filtered out.  Uniquely completing a branch in
a repository with 100k local branches, all packed, best of five:

  On Linux, before:

    $ time __git_complete_refs --cur=maste

    real    0m0.831s
    user    0m0.808s
    sys     0m0.028s

  After:

    real    0m0.119s
    user    0m0.104s
    sys     0m0.008s

  On Windows, before:

    real    0m1.480s
    user    0m1.031s
    sys     0m0.060s

  After:

    real    0m0.377s
    user    0m0.015s
    sys     0m0.030s

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

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