Transactions - fix inverse edge transaction writes

Summary: Ref XXXXX. I broke things a bit in XXXXX in that if the TYPE_EDGE had an inverse transaction, we weren't correctly "doing nothing" and instead were falling back to our old every editor has to implement a no-op ways... Fix things by putting the TYPE_EDGE code in the handle external builtin function like it belongs.

Test Plan: made a comment on a task referencng a commit successfully

Reviewers: epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T6403

Differential Revision: https://secure.phabricator.com/D12939