ls-refs.c: avoid enumerating hidden refs where possible

ls-refs.c: avoid enumerating hidden refs where possible

In a similar fashion as in previous commits, teach `ls-refs` to avoid
enumerating hidden references where possible.

As before, this is linux.git with one hidden reference per commit.

    $ hyperfine -L v ,.compile 'git{v} -c protocol.version=2 ls-remote .'
    Benchmark 1: git -c protocol.version=2 ls-remote .
      Time (mean ± σ):      89.8 ms ±   0.6 ms    [User: 84.3 ms, System: 5.7 ms]
      Range (min … max):    88.8 ms …  91.3 ms    32 runs

    Benchmark 2: git.compile -c protocol.version=2 ls-remote .
      Time (mean ± σ):       6.5 ms ±   0.1 ms    [User: 2.4 ms, System: 4.3 ms]
      Range (min … max):     6.2 ms …   8.3 ms    397 runs

    Summary
      'git.compile -c protocol.version=2 ls-remote .' ran
       13.85 ± 0.33 times faster than 'git -c protocol.version=2 ls-remote .'

Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>