diffhelper: rename module to avoid conflicts with ancient C module (issue5846)
Historically we had had C extensions in mercurial/, which shadows the pure
Python modules of the same name forever unless we do clean build/install.
I'm sloppy to think about new name, so just dropped the "s".
#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