parsers.c: parse_manifest: fixing refcount of flags
When flags was DECREF'ed, scope was referencing to the outer variable,
outside of the block.
It was in fact always NULL: the real Python object was never decref'ed.
http://example.com/no/anchor, revs: None, checkout: None
http://example.com/an/anchor, revs: ['foo'], checkout: 'foo'
http://example.com/no/anchor/revs, revs: ['foo'], checkout: 'foo'
http://example.com/an/anchor/revs, revs: ['foo', 'bar'], checkout: 'foo'
http://example.com/an/anchor/rev-None, revs: ['foo'], checkout: 'foo'