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
Add new commit method message-encoding
Message method now reads encodes correctly

Convert message encoding string to a symbol, this can then be used to
specify the correct encoding when converting the commit message to a
lisp native string.
Error if trying to use disconnected remote

If actions that require a remote connection are called when
disconnected, then raise an error.
Add #:flexi-streams to cl-git.asd

Miss #:flexi-streams will lead to an error "The name "FLEXI-STREAMS" does not designate any package".
Merge pull request #25 from muyinliu/patch-1

Add #:flexi-streams to cl-git.asd