Add $_prior, $_rebase_base, $_merge_base variables

* `$_prior` is like `@{-1}`, but easier to remember and search for. It also
  expands to the name of the prior branch, so it can be used for things like
  `push origin :$_prior`.

* `$_rebase_base` is the base onto which we are currently rebasing, e.g. for
  `rebase --onto foo bar baz` it would be the SHA1 of the head of the `foo`
  branch.

* `$_merge_base` is the base commit of the current merge. Equivalent to
  `merge-base HEAD MERGE_HEAD`.