Summary:
If edenfs was started using `sudo`, the `$USER` environment variable will be
set to `root` rather than the actual user. When we drop privileges make sure
we restore the value of `$USER` as well.
The `$USER` variable isn't checked anywhere else in edenfs itself, but it
matters for subprocesses we spawn, like `hg debugedenimporthelper`.
I also changed the code to clear the `SUDO_*` variables as well, mostly
just for good measure.
Reviewed By: kulshrax
Differential Revision: D15929539
fbshipit-source-id: e022c7ae762e2a5e86d0227058bb476aff17cf55
