Implement ObjectDatabase.ShortenObjectId()

Libgit2 **[exposes](https://github.com/libgit2/libgit2/blob/development/include/git2/object.h#L107-L114)** `git_object_short_id()` method. Let's make this available to LibGit2Sharp.

Proposed signature would be 

```csharp
string ObjectDatabase.ShortenObjectId(GitObject object)
```

Of course, if anyone comes up with a better name for this method, we'll be very grateful :wink: