branch3: use an explicitely experimental name for files
Since this format is still experimental, we don't want to have to side-step
the `branch3` name in case people do start using it before it's stable.
#if no-windows
$ 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