rebase: correctly check for empty commit in in-memory mode The new code has a small overhead in the empty commit case, as a `memctx` object is always created, but I don’t think it’s justified here to duplicate code to optimize a relatively unlikely code path. Differential Revision: https://phab.mercurial-scm.org/D8732

memctx: make `parents()` return list of one element if it’s not a merge It is part of the contract of `parents()` that non-merges return a list of one element. `self._parents` is always a list of two elements in `memctx`. Differential Revision: https://phab.mercurial-scm.org/D8731