When a webhook is received we synchronize the branches/tags to create new
versions under Read the Docs. For this process we do a full clone of the
repository and iterate over the branches/tags.

As this process of cloning the repository just for updating versions could be
expensive, this commit adds the ability to use `ls-remote` on Git VCS just to
query the repository for its branches/tags without performing a full clone.

When SyncRepositoryTaskStep calls `sync_repo` we decide if we need a full clone
of just listing the remote branches/tags to sync versions between Read the Docs
and the repository.