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

hgext/rebase.py

@@ -1102,13 +1102,14 b' def rebasenode(repo, rev, p1, base, stat'
     if wctx.isinmemory():
         wctx.setbase(repo[p1])
     else:
-        # This is necessary to invalidate workingctx's caches.
-        wctx = repo[None]
         if repo['.'].rev() != p1:
             repo.ui.debug(" update to %d:%s\n" % (p1, repo[p1]))
             mergemod.update(repo, p1, False, True)
         else:
             repo.ui.debug(" already in destination\n")
+        # This is, alas, necessary to invalidate workingctx's manifest cache,
+        # as well as other data we litter on it in other places.
+        wctx = repo[None]
         repo.dirstate.write(repo.currenttransaction())
     repo.ui.debug(" merge against %d:%s\n" % (rev, repo[rev]))
     if base is not None: