To accommodate less capable compilers.

Co-authored-by: Peter Pettersson <85582231+boretrk@users.noreply.github.com>

Looking up a non-existent object currently always invokes
`git_odb_refresh`. If looking up a large batch of objects, many of which
may legitimately not exist, this will repeatedly refresh the ODB to no
avail.

Add a `git_odb_exists_ext` that accepts flags controlling the ODB
lookup, and add a flag to suppress the refresh. This allows the user to
control if and when they refresh (for instance, refreshing once before
starting the batch).

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.