Coverage results inconsistent unless I delete old data

I have seen coverage reported as much as three percent higher if I re-run cover_me without deleting the previous data in the tmp/coverage/\* and coverage.data. A related symptom is that sorting on the number of untested lines of code does not work properly unless I delete the previous data.

Start by reproducing the issue with cover_me, first retaining and then deleting tmp/coverage/* and coverage.data. Inspect how those paths are read or updated; done means repeated runs report consistent coverage and sorting by untested lines works without manually deleting prior data.

Written by the indexing model from the issue text.