Fix pqueue sort boundary condition bug

If the pqueue comparison fn returned just 0 or 1 (think "a<b")
then the sort order of returned items could be wrong because there
was a "< 0" that really needed to be "<= 0".  Yikes!!!
Merge pull request #2184 from libgit2/rb/fix-revwalk-order-regression

Fix pqueue sort boundary condition bug