mergestate: make some callers not pass pointless node argument
The node argument is set on the created `mergestate` instance, but
these callers don't even look at that instance.
Differential Revision: https://phab.mercurial-scm.org/D9031
py3: don't risk passing a None value to error.ManifestLookupError()
This makes the test case added in
20dd2a259b0f (test-grep: add tests
for --follow with/without --diff and/or paths, 2020-09-11) pass on
Python 3.
Differential Revision: https://phab.mercurial-scm.org/D9030