Instead of making the commit and dump functions take individual options
structures; provide the options structure to the writer creator. This
allows us to add additional information (like OID type) during
generation.
This change adds the git_commit_graph_writer_* functions to allow to
write and create `commit-graph` files from `.idx`/`.pack` files or
`git_revwalk`s.
Part of: #5757
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.
