[github notification]: should refer to the revision of a specific sync operation instead of the current revision

Currently, when sending github notifications, it is always tied to [the current sync revision](https://github.com/argoproj/notifications-engine/blob/7b9b5d3281e1b52c17b04369de1cda8072d15072/pkg/services/github.go#L47). However, to be accurate, it should be tied to the revision for the specific revision, app.status.operationState.syncResult.revision

Start at pkg/services/github.go around the linked line and inspect how the GitHub notification currently obtains the revision. Trace app.status.operationState.syncResult.revision and verify that notifications refer to the specific sync operation's revision rather than the current revision.

Written by the indexing model from the issue text.