rust-nodemap-docket: move check of nodemap requirement to caller
I think it's cleaner if `NodeMapDocket` doesn't know about the `Repo`
type. That makes it more easily reusable and testable. This patch
moves out one of the uses of `Repo` out of it.
Differential Revision: https://phab.mercurial-scm.org/D12544
#require docutils gettext
$ $TESTDIR/check-gendoc it
checking for parse errors