convert: set LC_CTYPE around calls to Subversion bindings
The Subversion bindings require that LC_CTYPE is set. However, we don’t want to
set it all the time, as it changes the behavior of str methods on Python 2. The
taken approach is hopefully fine-grained enough to not trigger any
locale-specfic behavior of the str methods and coarse-grained enough to not
clutter the code.
Emulating the with-statement behavior in before() and after() should be safe, as
after() is always called when before() is called. hgext.convert.hg takes a