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

tests/test-xdg.t

@@ -0,0 +1,11 b''
+#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

mercurial/help/config.txt

@@ -56,6 +56,7 b' ones.'
 
   - ``<repo>/.hg/hgrc`` (per-repository)
   - ``$HOME/.hgrc`` (per-user)
+  - ``${XDG_CONFIG_HOME:-$HOME/.config}/hg/hgrc`` (per-user)
   - ``<install-root>/etc/mercurial/hgrc`` (per-installation)
   - ``<install-root>/etc/mercurial/hgrc.d/*.rc`` (per-installation)
   - ``/etc/mercurial/hgrc`` (per-system)

mercurial/scmposix.py

@@ -40,8 +40,15 b' def systemrcpath():'
 def userrcpath():
     if pycompat.sysplatform == 'plan9':
         return [encoding.environ['home'] + '/lib/hgrc']
+    elif pycompat.sysplatform == 'darwin':
+        return [os.path.expanduser('~/.hgrc')]
     else:
-        return [os.path.expanduser('~/.hgrc')]
+        confighome = encoding.environ.get('XDG_CONFIG_HOME')
+        if confighome is None or not os.path.isabs(confighome):
+            confighome = os.path.expanduser('~/.config')
+
+        return [os.path.expanduser('~/.hgrc'),
+                os.path.join(confighome, 'hg', 'hgrc')]
 
 def termsize(ui):
     try: