Fix Filesytem::walkToRoot() on Windows with default ("/") root

Summary:
Fixes T9459. See PHI817. An install is reporting that arc upgrade fails on Windows, failing to recognize that arcanist/ is a Git working copy.

This appears to reproduce for me, and boil down to FileList->contains() testing if / contains C:\Application Data32\Roaming\My Documents\Users\epriestley\devtools\arcanist. It (literally) does not, but our intent is that it should.

Instead of defaulting the root to /, treat it somewhat more specially so Filesystem::walkToRoot(X) on Windows always walks all the way up.

Test Plan: This is slightly shaky since my Windows "development environment" is a mess, but I was able to run arc upgrade with this patch applied and make it through working copy recognition. I'll re-test after landing.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T9459

Differential Revision: https://secure.phabricator.com/D19591