hg: allow usage of XDG_CONFIG_HOME/hg/hgrc
Modern applications must use the following paths to store configuration files:
- $XDG_CONFIG_HOME/hg/hgrc,
- $HOME/.config/hg/hgrc (if XDG_CONFIG_HOME is unset or not absolute).
For backward compatibility, ~/.hgrc is still created if no hgrc exist using hg
config --edit.
See https://standards.freedesktop.org/basedir-spec/basedir-spec-latest.html
#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
foobar
#endif