commitextras: check the format of the arguments and no internal key is used
This patch adds check to make the arguments are passed as KEY=VALUE and no key
which is used internally is passed.
This patch also adds test for the extension.
#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