R1:ef121b3e17cf

Fix a Herald repetition policy selection error for rule types which support only one policy

Summary:
Ref T13048. See <https://discourse.phabricator-community.org/t/configuring-commit-hook-commit-content-rules-fail-with-exception/1077/3>.

When a rule supports only one repetition policy (always "every time") like "Commit Hook" rules, we don't render a control for `repetition_policy` and fail to update it when saving.

Before the changes to support the new "if the rule did not match the last time" policy…