Update libgit2.
Link iconv if we're building on darwin.

I'm not really happy about this. libgit2's pkg-config file should specify
this in its Libs line, but it does not. :(
Update libgit2.
First pass at updating rugged to the new diff api changes.
Re-add line addition/deletion markers.
Make the `compact` option for Rugged::Diff#print work correctly again.
Rename `rugged_diff_patch_*` to `rugged_patch_*`
Rename `Rugged::Diff::Patch` to `Rugged::Patch`.
Remove unused variables.
Tiny code beautifications.
Merge pull request #277 from libgit2/arthur/update-diff-api

Update Rugged to the latest Diff API changes