tests: use pdiff instead of diff for portability
"diff" command might cause redundant message, "No differences
encountered" on Solaris for example. But suppressing option like "-q"
isn't portable, because POSIX specification doesn't define it.
pdiff script was introduced by b59ef0c21405 to stabilize output of
standard diff command on each platforms.
#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