Custom backends can allocate memory when reading objects and providing
them to libgit2.  However, if an error occurs in the custom backend
after the memory has been allocated for the custom object but before
it's returned to libgit2, the custom backend has no way to free that
memory and it must be leaked.

Provide a free function that corresponds to the alloc function so that
custom backends have an opportunity to free memory before they return an
error.

The `git_odb_backend_malloc` name is a system function that is provided
for custom ODB backends and allows them to allocate memory for an ODB
object in the read callback.  This is important so that libgit2 can
later free the memory used by an ODB object that was read from the
custom backend.

However, the name _suggests_ that it actually allocates a
`git_odb_backend`.  It does not; rename it to make it clear that it
actually allocates backend _data_.

Rename git_transfer_progress to git_indexer_progress

Safely deprecate `git_push_transfer_progress`, forwarding it to the new
`git_push_transfer_progress_cb` name.

The `git_push_transfer_progress` is a callback and as such should be
suffixed with `_cb` for consistency.  Rename
`git_push_transfer_progress` to `git_push_transfer_progress_cb`.

Safely deprecate `git_transfer_progress` and `git_transfer_progress_cb`
types, forwarding them to the new `git_indexer_progress` and
`git_indexer_progress_cb`.

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

The name `git_transfer_progress` does not reflect its true purpose.

It suggests that it's progress for a non-existence `git_transfer`
object, and is used for indexing callbacks more broadly than just
during transfers.

Rename `git_transfer_progress` to `git_indexer_progress`.

High-level map APIs

refdb_fs: fix loose/packed refs lookup racing with repacks