Fixes T13668

There are two issues which prevent cascading/rebasing of non-landed commits during a land,

1. ArcanistLandEngine repeatedly called cascadeState() passing in the last commit set and not any intermediary sets, resulting in failing to cascade branches made off of intermediate sets/revisions.
2. ArcanistMercurialLandEngine::cascadeState() unconditionally tried to rebase each child branch from a given set onto the last published commit.

To fix these,

1. Updated ArcanistLandEngine to pass in each set that needs cascaded.
2. For Mercurial repositories, whenever a merge is executed, keep a map of the old commit to the destination/squashed commit. During cascading, reference this map for two purposes:
   1. If a child branch from a commit set has a corresponding new commit from the squash, ignore/avoid cascading since it was part of the land process. Attempting to rebase this would result in rebase conflicts.
   2. For a child branch that does need cascaded, determine the appropriate destination commit so it rebases cleanly.

Mercurial:

1. I created 3 revisions, each with 2 commits and dependent on the previous revision.
2. From the middle revision I created a new branch of changes bookmarked fest and created a new revision for it.
3. I ran arc land test3 to land the newest/tip revision.
4. I verified that the original 3 revisions were landed properly as well as their original state being cleaned up.
5. I verified that the fest branch had been rebased onto the new commit for the middle revision.

TODO: same test with git