rebase: when collapsing, p1 == dest, so use the former only
`dest` is the destination we're rebasing onto, which is always the
same as `p1` when using `--collapse`. This lets us simplify a bit.
Differential Revision: https://phab.mercurial-scm.org/D9073
== New Features ==
* `hg mv -A` can now be used with `--at-rev`. It behaves just like
`hg cp -A --at-rev`, i.e. it marks the destination as a copy of the
source whether or not the source still exists (but the source must
exist in the parent revision).
== New Experimental Features ==
== Bug Fixes ==
== Backwards Compatibility Changes ==
== Internal API Changes ==