Add cvs as supported revision_type

This was recently added to swh.model

Test Plan

The hypothesis tests sometimes test this hypothesis

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Migrated from D6128 (view on Phabricator)

sql/upgrades/177.sql

+-- SWH DB schema upgrade
+-- from_version: 176
+-- to_version: 177
+-- description: add cvs to revision_type values
+
+insert into dbversion(version, release, description)
+      values(177, now(), 'Work In Progress');
+
+alter type revision_type add value 'cvs';