hglib issues under windows

[Daniel Santa Cruz]

Background:

Under Windows there is an issue with using os.path.join with strings
that tailor keeps.  The end result is paths with a combination of /
and \.

Lele suggested (in http://artiemestieri.tn.it/tailor/ticket/4) a
solution to this, which works great if using hg.py.

Problem:

The previous solution only works if one is using the 'hg' target and
not the 'hglib' target.  When using 'hglib', things are passed to
mercurial at a lower level, which does not "sanitize" paths.  At that
level, hg is expecting to see linux's pathsep (/).

If one removes the before-commit hook, then one ends up with / \ in
the same path.

Daniel