tests: remove unnecessary explicit args from `failfilemerge.py` The `failfilemerge()` function always raises an exception without looking at its arguments, so let's use `*args` and `**kwargs`. Differential Revision: https://phab.mercurial-scm.org/D11854

simplemerge: set `conflicts` earlier for :union tool Differential Revision: https://phab.mercurial-scm.org/D11853

downgrade: don't assume existence of nodemap files when downgrading Persistent nodemap files might not exist on a fresh repository, so ignore the potential missing file errors. Differential Revision: https://phab.mercurial-scm.org/D11806