logexchange: convert paths to unix when detecting the active path
This fixes the problem in the tests[1] where Windows was showing the whole path
as the remotename for local repositories.
Somebody with a better understanding of this extension should probably take a
deeper look. There may be other cases that need to be converted- specifically
the `elif not instance` and the missing `else` cases in activepath(). I also
noticed when adding debug prints that the absolute path is stored in the file,
probably not normalized. (It's wrapped up in $TESTTMP.)
[1] https://buildbot.mercurial-scm.org/builders/Win7%20x86_64%20hg%20tests/builds/1042/steps/run-tests.py%20%28python%202.7.13%29/logs/stdio