Improve handling on monthly/yearly scheduling.

Code was still buggy, it turns out (though the recursion checker caught
it). In the case of (Schedule (Monthly Nothing) AnyTime), where the last
run was on yyyy-12-31, it looped forever.

Also, the handling of (Schedule (Yearly Nothing) AnyTime) was wacky where
the last run was yyyy-12-31. It would suggest a window starting on the 3rd
for the next run (because 31 mod 28 is 3).

I think that originally I was wanted to avoid running on 01-01 if it had
just run on 12-31. But the code didn't accomplish this, and it's not
necessary anyway. This is supposed to calculate the next window meeting the
schedule, and for (Schedule (Monthly Nothing), the window starts at 01-01
and runs through 01-31. If that causes two back-to-back runs, well the next
one will not be until 02-01 at the earliest.

Also, back-to-back runs can be avoided, if desired, by using Divisible 2.

debian/changelog

@@ -1,3 +1,9 @@
+git-annex (5.20140413) UNRELEASED; urgency=medium
+
+  * Improve handling on monthly/yearly scheduling.
+
+ -- Joey Hess <joeyh@debian.org>  Fri, 11 Apr 2014 21:33:35 -0400
+
 git-annex (5.20140412) unstable; urgency=high
 
   * Last release didn't quite fix the high cpu issue in all cases, this should.