pack-objects: keep track of `pack_start` for each reuse pack

When reusing objects from a pack, we keep track of a set of one or more
`reused_chunk`s, corresponding to sections of one or more object(s) from
a source pack that we are reusing. Each chunk contains two pieces of
information:

  - the offset of the first object in the source pack (relative to the
    beginning of the source pack)
  - the difference between that offset, and the corresponding offset in
    the pack we're generating

The purpose of keeping track of these is so that we can patch an
OFS_DELTAs that cross over a section of the reuse pack that we didn't
take.

For instance, consider a hypothetical pack as shown below:

                                                (chunk #2)
                                                __________...
                                               /
                                              /
      +--------+---------+-------------------+---------+
  ... | <base> | <other> |      (unused)     | <delta> | ...
      +--------+---------+-------------------+---------+
       \                /
        \______________/
           (chunk #1)

Suppose that we are sending objects "base", "other", and "delta", and
that the "delta" object is stored as an OFS_DELTA, and that its base is
"base". If we don't send any objects in the "(unused)" range, we can't
copy the delta'd object directly, since its delta offset includes a
range of the pack that we didn't copy, so we have to account for that
difference when patching and reassembling the delta.

In order to compute this value correctly, we need to know not only where
we are in the packfile we're assembling (with `hashfile_total(f)`) but
also the position of the first byte of the packfile that we are
currently reusing. Currently, this works just fine, since when reusing
only a single pack those two values are always identical (because
verbatim reuse is the first thing pack-objects does when enabled after
writing the pack header).

But when reusing multiple packs which have one or more gaps, we'll need
to account for these two values diverging.

Together, these two allow us to compute the reused chunk's offset
difference relative to the start of the reused pack, as desired.

Helped-by: Jeff King <peff@peff.net>
Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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