test-tool: add pack-deltas helper

When trying to demonstrate certain behavior in tests, it can be helpful
to create packfiles that have specific delta structures. 'git
pack-objects' uses various algorithms to select deltas based on their
compression rates, but that does not always demonstrate all possible
packfile shapes. This becomes especially important when wanting to test
'git index-pack' and its ability to parse certain pack shapes.

We have prior art in t/lib-pack.sh, where certain delta structures are
produced by manually writing certain opaque pack contents. However,
producing these script updates is cumbersome and difficult to do as a
contributor.

Instead, create a new test-tool, 'test-tool pack-deltas', that reads a
list of instructions for which objects to include in a packfile and how
those objects should be written in delta form.

At the moment, this only supports REF_DELTAs as those are the kinds of
deltas needed to exercise a bug in 'git index-pack'.

Signed-off-by: Derrick Stolee <stolee@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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