fixed api changes for index.c

11d9f6b304 in libgit2 changed api for git_index_find()
variable renaming
added GIT_EINVALIDSPEC as error code and use error msg for expected failures
fix error message text
docs: document arguments in a few more methods
Revert mistake
Revert another mistake

Forgot the Python 3 changes to the API.
Add Python 3.3 to the list of supported versions
docs: finish first round on the repository chapter