Fix ahead-behind tests

The logic was reversed. I have checked manually each pair with git and
adjusted the expectation to what git status prints.
graph: fix ahead-behind logic

When we see PARENT1, it means there is a local commit and thus we are
ahead. Likewise, seeing PARENT2 means that the upstream branch has a
commit and we are one more behind.

The logic is currently reversed. Correct it.

This fixes #2501.
Merge pull request #2539 from libgit2/cmn/ahead-behind-order

Fix ahead-behind results