debuginstall: use codecs.lookup() to detect invalid encoding
encoding.fromlocal() never tries to decode an ascii string since
853574db5b12,
and there's no universal non-ascii string which can be decoded as any valid
character set.
#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