rebase: fix for hgsubversion 5c25fe7fb1e broke something in the hgsubversion test path, causing it raise an abort (Abort: nothing to merge) during a perfectly good rebase. I tracked it down to this change. It's probably not hgsubversion related. I suspect that using the same `wctx` from before the initial update causes problems with the wctx's cached manifest property. I noticed we also sometimes stick random gunk on the wctx object in other places (like in `copies.py`) so it's probably best to reset it for now. The line I added before was actually useless since we don't pass wctx to the initial `merge.update`, so it defaults to `repo[None]`. So I just removed it. Differential Revision: https://phab.mercurial-scm.org/D1679