Fix regression with custom arguments being dropped in non-local… (#491)

* No idea what I'm doing.

* Extend integration test to cover ac3f6a78c7401aba9b62c2c67f280a129ae84eaf.

* Remove useless output to stop failing other tests.

* Reformat.

* Upgrade virtualenv to see if it fixes py34 builds

* Add CHANGELOG entry

* Make foobarplugin part of the test only

* Fix language_version in pre-commit for rst

* Fix test in Python 2

Co-authored-by: Bruno Oliveira <nicoddemus@gmail.com>

src/xdist/workermanage.py

@@ -190,6 +190,7 @@ def make_reltoroot(roots, args):
         parts = arg.split(splitcode)
         fspath = py.path.local(parts[0])
         if not fspath.exists():
+            result.append(arg)
             continue
         for root in roots:
             x = fspath.relto(root)