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