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
$ hg init
$ echo This is file a1 > a
$ hg add a
$ hg commit -m "commit #0"
$ ls
a
$ echo This is file b1 > b
$ hg add b
$ hg commit -m "commit #1"
$ hg co 0
0 files updated, 0 files merged, 1 files removed, 0 files unresolved
B should disappear
$ ls
a