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>