Updated all copyright headers
Convert file mode int to keywords
Export the correct flag in the diff struct

In some situations it won't have a value, hence the nil.
Fix invalid references to old symbols

Updated free methods to match new names in version 0.20.0
- git-diff-list-free to git-diff-free
- git-diff-patch-free to git-patch-free
Fixed up some diff methods

(DIFF TREE INDEX) and (DIFF REPOSITORY INDEX) now both should work.
Hack garbage collection on for the diff API