rewriteutil: make sure to provide branch kwarg to tomemctx()

Earlier in this function we did extra[b'branch'] = head.branch(), so we definitely want to take branch from head. But inside tomemctx(), there's an if branch is None check that updates extra with wctx.branch(). That is not what we need, so let's provide the correct branch again, as a separate keyword argument, even though it's already correct in extra.

Other uses of tomemctx() already provide branch.

This patch could be on stable, but fixup series depends on it, and we're close enough to a feature release, so this should be fine.