Cleanup FilterBranchFixture test repositories
Update libgit2 binaries to 711333e

https://github.com/libgit2/libgit2/compare/11f8336...711333e
Ensure shadow copying doesn't prevent from probing
Introduce RequiresDotNetOrMonoGreaterThanOrEqualTo() helper
Make RetrieveStatus() reload on-disk index beforehand

Fix #322 and #522
Enforce creation of unborn branch test coverage