Share Env/CWD methods between Passthru + ExecFuture, expose Env

Summary:
Ref T10751. This is several layers deep: I want to unit test environmental configuration for git/svn/hg commands, but there's currently no way to read it.

It's also duplicated between Passthru and ExecFuture classes.

Extend them from one class doing CWD + Env stuff. Clean that stuff up a little. Add a getEnv() for reading so I can do unit tests.

Test Plan: Ran arc unit --everything, see next diff for unit tests.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10751

Differential Revision: https://secure.phabricator.com/D15751