Reuse libgit2 test repos.

I changed `sandbox_init` to always use the test repos coming from libgit2, and removed our copies from `test/fixtures`. There's still some tests that use some of the test repos that are not found in libgit2 - I'm not really sure what to do about those.

@brianmario: @vmg and me have been thinking about dropping the tests from the gem, as including all the test repos inside the gem bloats it by a megabyte or so. What do you think?