Fixed crashes in `Repository.create_remote` caused by missing `Py_INCREF()`
refactoring Repository_git_reference_symbolic_create
added basic diff example and fixed misleading doc
Merge remote-tracking branch 'jun/crash-in-create-remote'
Merge remote-tracking branch 'cholin/features/diff_docs'
Merge remote-tracking branch 'cholin/features/refactoring'
Sort list of authors by number of commits

As shown by "git shortlog -sn". So those who have done more are
more visible.