adjustlinkrev: prepare source revs for ancestry only once We'll need some more complex initialisation to handle workingfilectx case. We do this small change in a different patch for clarity.

subrepo: add the parent context to hgsubrepo This brings parity with gitsubrepo and svnsubrepo (which already reference their parent), and will be used in an upcoming patch. I'm a bit concerned that the parent context could get stale (consider what happens when the parent repo is reverted for example). I tried adding the parent context to the substate tuple so that the parent is available everywhere a state change is possible, but that made submerge() unhappy. Even with removing the parent context inside submerge(), I wasn't able to get all of the test diffs fixed. But since the other subrepos reference their parent too, if there is a problem, it is a preexisting one (that nobody seems to be running into). It can be fixed if/when it pops up.