make readHash() check for undefined
Make objects.read() check for undefined
Add objects.readTree() to read git tree from obj db
Add files.flattenNestedTree() to unnest dir tree

* From path:to:file:content to path:content
Add diff.readCommitIndex()

* Takes a hash to a git tree and turns it into an index
rm broken checkout() code
Make diff() ref checks support hashes
Add first run at diff(ref, undefined)
Pin date for diff tests
Test can pass HEAD or branch name to diff()
Don't test extra stuff
Move diff HEAD tests into own describe block