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
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