completion: fill COMPREPLY directly when completing refs

__gitcomp_nl() iterates over all the possible completion words it gets
as argument

  - filtering matching words,
  - appending a trailing space to each matching word (in all but two
    cases),
  - prepending a prefix to each matching word (when completing words
    after e.g. '--option=<TAB>' or 'master..<TAB>'), and
  - adding each matching word to the COMPREPLY array.

This takes a while when a lot of refs are passed to __gitcomp_nl().

The previous changes in this series ensure that __git_refs() lists
only refs matching the current word to be completed, making a second
filtering in __gitcomp_nl() redundant.

Adding the necessary prefix and suffix could be done in __git_refs()
as well:

  - When refs come from 'git for-each-ref', then that prefix and
    suffix could be added much more efficiently using a 'git
    for-each-ref' format containing said prefix and suffix.  Care
    should be taken, though, because that prefix might contain
    'for-each-ref' format specifiers as part of the left hand side of
    a '..' range or '...' symmetric difference notation or
    fetch/push/etc. refspec, e.g. 'git log "evil-%(refname)..br<TAB>'.
    Doubling every '%' in the prefix will prevent 'git for-each-ref'
    from interpolating any of those contained specifiers.
  - When refs come from 'git ls-remote', then that prefix and suffix
    can be added in the shell loop that has to process 'git
    ls-remote's output anyway.
  - Finally, the prefix and suffix can be added to that handful of
    potentially matching symbolic and pseudo refs right away in the
    shell loop listing them.

And then all what is still left to do is to assign a bunch of
newline-separated words to a shell array, which can be done without a
shell loop iterating over each word, basically making all of
__gitcomp_nl() unnecessary for refs completion.

Add the helper function __gitcomp_direct() to fill the COMPREPLY array
with prefiltered and preprocessed words without any additional
processing, without a shell loop, with just one single compound
assignment.  Modify __git_refs() to accept prefix and suffix
parameters and add them to each and every listed ref as described
above.  Modify __git_complete_refs() to pass the prefix and suffix
parameters to __git_refs() and to feed __git_refs()'s output to
__gitcomp_direct() instead of __gitcomp_nl().

This speeds up refs completion when there are a lot of refs matching
the current word to be completed.  Listing all branches for completion
in a repo with 100k local branches, all packed, best of five:

  On Linux, near the beginning of this series, for reference:

    $ time __git_complete_refs

    real    0m2.028s
    user    0m1.692s
    sys     0m0.344s

  Before this patch:

    real    0m1.135s
    user    0m1.112s
    sys     0m0.024s

  After:

    real    0m0.367s
    user    0m0.352s
    sys     0m0.020s

  On Windows, near the beginning:

    real    0m13.078s
    user    0m1.609s
    sys     0m0.060s

  Before this patch:

    real    0m2.093s
    user    0m1.641s
    sys     0m0.060s

  After:

    real    0m0.683s
    user    0m0.203s
    sys     0m0.076s

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

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