Contexts in reports are missing with relative_files=True

[lazka]

Describe the bug

When I enable relative_files the json and html reports no longer contain context information. I still see context info in the sqlite DB though.

To Reproduce

• coverage.py 5.0, py3.7.5
• Set relative_files=True
• python3 -m pytest --cov --cov-context=test tests/test_foo.py
• python3 -m coverage html --show-contexts
• HTML report doesn't include any context infos

Expected behavior

HTML report contains context infos

[nedbat]

I'll mark this a dup of #900.

[nedbat]

This is now available in coverage==5.0.1