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.