run-tests: make code coverage work on Python 3
This code path was obviously not tested on Python 3 because it
blew up in several places due to str/bytes mismatch.
For internal code, we normalize paths to bytes.
For code calling into `coverage`, we normalize paths to str,
which is what `coverage` seems to expect.
After this, `run-tests.py -H` works on Python 3!
Differential Revision: https://phab.mercurial-scm.org/D7133
Set up
$ hg init repo
$ cd repo
Try to import an empty patch
$ hg import --no-commit - <<EOF
> EOF
applying patch from stdin
abort: stdin: no diffs found
[255]
No dirstate backups are left behind
$ ls .hg/dirstate* | sort
.hg/dirstate