Fix paths when running coverage from root

[arthurio]

I'm running coverage from a docker image where my source code is at /. I couldn't figure out why the paths were all starting with / but not if I moved things in a subfolder (e.g. /subfolder). With the current implementation, RELATIVE_DIR would be equal to // instead of / because we suffix no matter what.

I tested this change manually in my docker image with [email protected] and coverage==6.0.2.

[arthurio]

The simple tests I added are heavily mocked... let me know if you can think of something better!

[nedbat]

Thanks. I see what you mean about the mocking, but i don't see a better way.

[nedbat]

This is now released as part of coverage 6.4.3.