Code review comment for lp:~spiv/bzr/ids-only-necessary-texts

218 + if not self.repository_format.supports_external_lookups:
219 + raise TestNotApplicable("Need stacking support in the source.")
220 + if not self.repository_format_to.supports_external_lookups:
221 + raise TestNotApplicable("Need stacking support in the target.")

It would be good to ensure that there is a compatible pair being tested. Otherwise we'll get TNA but not run the test.

Also, the 'get parents of the edge of a graph subset' is a repeated idiom now. I think its time to put that into graph.