Add append_log to reference.
Merge remote-tracking branch 'xtao/reflog'
Make Reference.log_append signature more consistent

Consistent with the rest of the pygit2 API.
Adjust to libgit2 development branch

This wraps the previous functionality, though there are some iterator
changes we might still want to bring over.
Branch: move from foreach to the iterator
Index: adjust to index_read() force flag