.arcconfig

rebase: set `prepared = True` at very end of `_preparenewrebase()`

Once we've set `rebaseruntime.prepared = True`, `rebaseruntime.repo`
starts returning the unfiltered repo. That will make my next patch
break, because that patch moves the call to `rewriteutil.precheck()`
after the call to `_handleskippingobsolete()`, which current happens
after `prepared = True`. We therefore need to prepare by moving
`prepared = True` a bit later, after `_handleskippingobsolete()`. I
don't think that matters for that call.

Differential Revision: https://phab.mercurial-scm.org/D10257

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