we need to allow passing nil for either side of the diff
add Rugged::Tree#diff_workdir
Make it so the left or right side of Rugged::Tree#diff can be nil

This is how you'd get a diff of a parentless commit, for example.
let nil through, heh
Remove some typos and update docs.
Add docs for #diff_workdir.
Update docs for Rugged::Commit#diff.
Added Rugged::Commit#workdir_diff.
Rugged::Tree#diff should always take at least one parameter.
Add some tests for diffing against an empty tree.
Make Rugged::Commit#diff return a diff against its parent by default.
Allow passing only an options hash to Rugged::Commit#diff.
This is no longer valid.
Simplify Rugged::Repository#diff and Rugged::Repository#diff_workdir
Make Rugged::Repository#diff work with OID strings.
Allow passing a rev string to Rugged::Tree#diff.
Fix a potential memory leak.
Use revparse instead of lookup.
Also allow passing rev strings to Rugged::Repository#diff_workdir.
Merge branch 'development' of git://github.com/libgit2/rugged into diff-changes