rebase: use rewriteutil.precheck() instead of reimplementing it After this patch, there's still another place in `rebase.py`, in the `--stop` code path, that reimplements `rewriteutil.precheck()`. I couldn't fix that place because it `rewriteutil.precheck()` checks that there is only one dirstate parent, which fails because we have two parents at that point. I think it's incorrect that rebase leaves the user with two parents during conflicts, but changing that is way out of scope for this series. Differential Revision: https://phab.mercurial-scm.org/D7685