Update libgit2, tests-clar/ -> tests/
Add Patch#to_s.
Merge pull request #289 from libgit2/arthur/patch-to-s

Add Patch#to_s.
Update to latest libgit2.
Merge pull request #291 from libgit2/arthur/update-libgit2

Update to latest libgit2
Expose the content_offset from libgit2 diff lines