Ensure that just reading a manifest file doesn't require us to be in a git repo.
Ensure that just reading a manifest file doesn't require us to be in a git repo.

This regressed with ato's recent changes. In general it isn't OK to assume that
web-platform-tests are actually running in a git repo rather than in a copy
except when doing operations that actually require a git operation. Therefore
trying to find the repo root at the top level isn't OK.

frewsxcv/wpt-tools,wpt-on-tv-tf/wpt-tools,wpt-on-tv-tf/wpt-tools,vivliostyle/wpt-tools,wpt-on-tv-tf/wpt-tools,frewsxcv/wpt-tools,vivliostyle/wpt-tools,UprootStaging/wpt-tools,UprootStaging/wpt-tools,kaixinjxq/wpt-tools,UprootStaging/wpt-tools,frewsxcv/wpt-tools,kaixinjxq/wpt-tools,vivliostyle/wpt-tools,kaixinjxq/wpt-tools