Cannot run swift-format from root (`/`)

[bnbarham]

Running swift-format from the root directory results in the first couple characters being dropped from path, eg. on macOS:

swift-format lint --recursive "/tmp/packagetest"
<unknown>: error: Unable to open rivate/tmp/packagetest/Package.swift: file is not readable or does not exist

rdar://134768169

[macshome]

Tracked in #862

[kkebo]

#865 has been merged.