Add ability to diff two arbitrary buffers

[APErebus]

libgit2 exposes a method to perform diff's between arbitrary buffers, git_diff_buffers.

It would be nice to expose this so that consumers can generate git-compatible diffs from arbitrary buffers.

I have exposed this call as two static methods on the Diff class (this felt like the most appropriate place, but I'm not 100% sure).

• public static ContentChanges Compare(byte[] oldBuffer, byte[] newBuffer)
• public static ContentChanges Compare(byte[] oldBuffer, byte[] newBuffer, CompareOptions compareOptions)

There is a new NativeMethod call and Proxy for git_diff_buffers and a new constructor for ContentChanges that executes this call when provided with two byte arrays.

I have also added a test suite that verifies that it produces the same output as git as well as the same tests as the Blob to Blob diff tests