manifestv2: set requires at repo creation time
While it should be safe to switch to the new manifest format on an
existing repo, let's keep it simple for now and make the configuration
have any effect only at repo creation time. If the configuration is
enabled then (at repo creation), we add an entry to requires and read
that instead of the configuration from then on.
% creating repo
% add and commit
C test.py
% change
M test.py