repack: introduce new compilation unit

repack: introduce new compilation unit

Over the years, builtin/repack.c has turned into a grab-bag of
functionality powering the 'git repack' builtin. Among its many
capabilities, it:

 - can build and spawn 'git pack-objects' commands, which in turn
   generate new packs
 - has infrastructure to manage the set of existing packs in a
   repository
 - has infrastructure to split a sequence of packs into a geometric
   progression based on object size
 - can manage both generating and combining cruft packs together
 - can write new MIDXs

to name a few.

As a result, this builtin has accumulated a lot of code, making adding
new functionality difficult. In the future, 'repack' will learn how to
manage a chain of incremental MIDXs, adding yet more functionality into
the builtin.

As a prerequisite step, let's first move some of the functionality in
the builtin into its own repack.[ch].

This will be done over the course of many steps, since there are many
individual components, some of which will end up in other, yet-to-exist
compilation units of their own. Some of the code movement here is also
non-trivial, so performing it in individual steps will make it easier to
verify.

Let's start by migrating 'struct pack_objects_args' (and the related
corresponding pack_objects_args_release() function) into repack.h, and
teach both the Makefile and Meson how to build the new compilation unit.

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