Update to new release

[carlosmn]

Adjust to the changes in 0.20. This includes moving away from callback to iterations, letting us do better error reporting.

[carlosmn]

While doing this update, I noticed that pygit2 returns a tuple when asked for the references in a repo. It seems really odd to me to return a tuple rather than a list, as you don't know how many there are beforehand, and they're not sorted (and sorting wouldn't have any meaning you could put into a tuple anyway). Is there some deeper pythonic reason for this?

[jdavid]

You're right, it would be more correct, pythonic, to return a list.

The Python criteria is, if it is a sequence of semantically the same kind of object, then it is a list. If it is a package of different kind of objects, like a record, then it is a tuple.