rust: Move VFS code to its own module
It was previously in the hg::repo module, but both repo code and vfs
will likely grow in the future.
Differential Revision: https://phab.mercurial-scm.org/D11394
rhg: Switch rhg.ignored-extensions config to Python-compatible list syntax
This includes the ability to have an extension name/path that contains a comma,
by double-quoting it.
Differential Revision: https://phab.mercurial-scm.org/D11390