ref-filter: parse objects on demand

When formatting an arbitrary object we parse that object regardless of
whether or not we actually need any parsed data. In fact, many of the
atoms we have don't require any.

Refactor the code so that we parse the data on demand when we see an
atom that wants to access the objects. This leads to a small speedup,
for example in the Chromium repository with around 40000 refs:

    Benchmark 1: for-each-ref --format='%(raw)' (HEAD~)
      Time (mean ± σ):     388.7 ms ±   1.1 ms    [User: 322.2 ms, System: 65.0 ms]
      Range (min … max):   387.3 ms … 390.8 ms    10 runs

    Benchmark 2: for-each-ref --format='%(raw)' (HEAD)
      Time (mean ± σ):     344.7 ms ±   0.7 ms    [User: 287.8 ms, System: 55.1 ms]
      Range (min … max):   343.9 ms … 345.7 ms    10 runs

    Summary
      for-each-ref --format='%(raw)' (HEAD) ran
        1.13 ± 0.00 times faster than for-each-ref --format='%(raw)' (HEAD~)

With this change, we now spend ~90% of the time decompressing objects,
which is almost as good as it gets regarding git-for-each-ref(1)'s own
infrastructure.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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