Introduce some tests that show some commits, while hiding some commits
that have a timestamp older than the common ancestors of these two
commits.

Only provide the empty tree internally, which matches git's behavior.
If we provide the empty blob then any users trying to write it with
libgit2 would omit it from actually landing in the odb, which appear
to git proper as a broken repository (missing that object).

Since writing multiple objects may all already exist in a single
packfile, avoid freshening that packfile repeatedly in a tight loop.
Instead, only freshen pack files every 2 seconds.

When writing an object, we calculate its OID and see if it exists in the
object database.  If it does, we need to freshen the file that contains
it.

The old implementation had two issues:

1. OIDs that were too short as to be ambiguous were not being handled
properly.

2. If the last OID to expand in the array was missing from the ODB, we
would leak a `GIT_ENOTFOUND` error code from the function.

Take (and write to) an array of a struct, `git_odb_expand_id`.

Query the object database for multiple objects at a time, given their
object ID (which may be abbreviated) and optional type.

For most real use cases, repositories with alternates use them as main
object storage. Checking the alternate for objects before the main
repository should result in measurable speedups.

Because of this, we're changing the sorting algorithm to prioritize
alternates *in cases where two backends have the same priority*. This
means that the pack backend for the alternate will be checked before the
pack backend for the main repository *but* both of them will be checked
before any loose backends.

As refdb and odb backends can be allocated by client code, libgit2
can’t know whether an alternative memory allocator was used, and thus
should not try to call `git__free` on those objects.

Instead, odb and refdb backend implementations must always provide
their own `free` functions to ensure memory gets freed correctly.