It looks like libgit2 supports changing context lines, but currently LibGit2Sharp is hard-coded to use 3: https://github.com/libgit2/libgit2sharp/blob/6a2d99ecdf35288df88c0e6fe8985969042d82a6/LibGit2Sharp/Diff.cs#L27
I've created https://github.com/libgit2/libgit2sharp/issues/423 to track the feature request.
Update:
As of v0.12 (or whatever comes after v0.11), you can do this:
var co = new CompareOptions
{
ContextLines = 0,
};
var tc = repo.Diff.Compare(new[] { filename }, co);