Fix bug in parsing of parens in some preferred content expressions. This fixes the behavior of the manual mode group.

The current manual mode preferred content expression is:

"present and (((exclude=*/archive/* and exclude=archive/*) or (not (copies=archive:1 or copies=smallarchive:1))) or (not copies=semitrusted+:1))"

The old matcher misparsed this, to basically:

OR (present and (...)) (not copies=semitrusted+:1))

The paren handling and indeed the whole conversion from tokens to the
matcher was just wrong. The new way may not be the cleverest, but I think
it is correct, and you can see how it pattern matches structurally against
the expressions when parsing them.

That expression is now parsed to:

MAnd (MOp <function>)
  (MOr (MOr (MAnd (MOp <function>) (MOp <function>)) (MNot (MOr (MOp <function>) (MOp <function>))))
    (MNot (MOp <function>)))

Which appears correct, and behaves correct in testing.

Also threw in a simplifier, so the final generated Matcher has less
unnecessary clutter in it. Mostly so that I could more easily read &
confirm them.

Also, added a simple test of the Matcher to the test suite.

There is a small chance of badly formed preferred content expressions
behaving differently than before due to this rewrite.

debian/changelog

@@ -9,6 +9,8 @@ git-annex (4.20130522) UNRELEASED; urgency=low
   * hook special remote: Added combined hook program support.
   * Android app: Avoid using hard links to app's lib directory, which
     is sometimes on a different filesystem than the data directory.
+  * Fix bug in parsing of parens in some preferred content expressions.
+    This fixes the behavior of the manual mode group.
 
  -- Joey Hess <joeyh@debian.org>  Tue, 21 May 2013 18:22:46 -0400