legacy: fix debugrecordpruneparents to call obsstore.create() with keywords
It appears the API was changed twice in Mercurial at
- 570f87422f54 "obsstore: add an explicit `date` argument to obsstore.create"
- adb3798dce49 "obsstore: add a `parents` argument to obsstore.create"
and metadata would be changed to a list of (key, value) pairs. Convert it back
to a dict as expected by create().