packfile: skip unpacking object header for disk size requests

While most of the object info requests for a packed object require us to
unpack its headers, reading its disk size doesn't. We still unpack the
object header in that case though, which is unnecessary work.

Skip reading the header if only the disk size is requested. This leads
to a small speedup when reading disk size, only. The following benchmark
was done in the Git repository:

    Benchmark 1: ./git rev-list --disk-usage HEAD (rev = HEAD~)
      Time (mean ± σ):     105.2 ms ±   0.6 ms    [User: 91.4 ms, System: 13.3 ms]
      Range (min … max):   103.7 ms … 106.0 ms    27 runs

    Benchmark 2: ./git rev-list --disk-usage HEAD (rev = HEAD)
      Time (mean ± σ):      96.7 ms ±   0.4 ms    [User: 86.2 ms, System: 10.0 ms]
      Range (min … max):    96.2 ms …  98.1 ms    30 runs

    Summary
      ./git rev-list --disk-usage HEAD (rev = HEAD) ran
        1.09 ± 0.01 times faster than ./git rev-list --disk-usage HEAD (rev = HEAD~)

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

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