mergestate: add accessors for local and other nodeid, not just contexts
The mergestate can contain invalid nodeids. In that case,
`mergestate.localctx` or `mergestate.otherctx` will fail. This patch
provides a way of accessing the nodeid without failing in such cases.
Differential Revision: https://phab.mercurial-scm.org/D8040
Run Mercurial tests with Vagrant:
$ vagrant up
$ vagrant ssh -c ./run-tests.sh