legacy-revlog: fix requirement computation when cloning legacy repo The oldest format of repository does not have requirements so we need to treat them differently when cloning such repository. The previous code had issue whenever we start using a working-copy-only requirements. The "legacy" format is signaled using an empty requirements list. If we add working-copy-only requirements to it, this is no longer empty, and no longer legacy. The new code fix this, and will get fully tested in a couple of changeset, once the share-safe become the default. Differential Revision: https://phab.mercurial-scm.org/D11995