Detect issues coming from a parent revision

[vrigal]

Ref. #1581 (comment)

That feature was lost with the before/after implementation because issues detected outside the diff are now published (when the feature runs).
I is possible to use the IssueList endpoint to list issues matching the repository/parent revision changeset, then explicitly display it in Phabricator.
Parent revisions can be retrieved via Phabricator libmozdata client - api.load_parents(revision.phabricator_phid).

[marco-c]

The implementation for this should be similar to the one we have to check differences in issues between diffs in the same revision.