largefiles: replace readstandin() by readasstandin() These code paths already (or should, for efficiency at repetition) know the target changectx and path of standin file.

largefiles: introduce readasstandin() to read hex hash from given filectx This will be used to centralize and encapsulate the logic to read hash from given (filectx of) standin file. readstandin() isn't suitable for this purpose, because there are some code paths, which want to read hex hash directly from filectx.

rebase: abort hg pull --rebase if rebase.requiredest is set (issue5514) Previously, the pull would succeed, but the subsequent rebase would fail due to the rebase.requiredest flag. Now abort earlier with a more useful error message.

rebase: demonstrate behavior with requiredest and pull --rebase

rebase: allow destination-free continue and abort (issue5513)

rebase: test to show brokenness with requiredest As shown in issue5513, --continue is broken when destination is required. This adds a patch that demonstates this silly behavior, which will be fixed in a future patch.

rebase: move destination test to new test file We'll be adding a lot more tests here, so it makes sense to have this in its own file now.