tests/test-xdg.t

rust-filepatterns: use bytes instead of String

In my initial patch, I introduced an unnecessary hard constraint on UTF-8
filenames and patterns which I forgot to remove. Although the performance
 penalty for using String might be negligible, we don't want to break
compatibility with non-UTF-8 encodings for no reason.
Moreover, this change allows for a cleaner Rust core API.

This patch introduces a new utils module that is used with this fix.

Finally, PatternError was not put inside the Python module generated by
Rust, which would have raised a NameError.

Differential Revision: https://phab.mercurial-scm.org/D6485

#if no-windows no-osx

  $ mkdir -p xdgconf/hg
  $ echo '[ui]' > xdgconf/hg/hgrc
  $ echo 'username = foobar' >> xdgconf/hg/hgrc
  $ XDG_CONFIG_HOME="`pwd`/xdgconf" ; export XDG_CONFIG_HOME
  $ unset HGRCPATH
  $ hg config ui.username 2>/dev/null
  foobar

#endif