This change introduces git_odb_write_multi_pack_index(), which creates a
`multi-pack-index` file from all the `.pack` files that have been loaded
in the ODB.

Fixes: #5399

commit-graph: Use the commit-graph in revwalks

The return code description use '-' char as list of return value.
But with the generation of the documentation the char '-' create an
ambiguous return code value who seem an negative value.

This change does a medium-size refactor of the git_commit_graph_file and
the interaction with the ODB. Now instead of the ODB owning a direct
reference to the git_commit_graph_file, there will be an intermediate
git_commit_graph. The main advantage of that is that now end users can
explicitly set a git_commit_graph that is eagerly checked for errors,
while still being able to lazily use the commit-graph in a regular ODB,
if the file is present.

Instead of using a signed type (`off_t`) use a new `git_object_size_t`
for the sizes of objects.

Docurium seems to choke on this header because it can't see both
git_indexer_progress & git_indexer_progress_cb, let's add the include.

Update internal usage of `git_transfer_progress` to
`git_indexer_progreses`.

Since we now always build the library with cdecl calling conventions,
our callbacks should be decorated as such so that users will not be able
to provide callbacks defined with other calling conventions.

The `GIT_CALLBACK` macro will inject the `__cdecl` attribute as
appropriate.

git_object_t is the future; update the public API to use it.  This will
also ensure that we can build our tests which make use of the old API
without modification (and without compiler warnings).