Teach Checkout() to cope with revparse syntax
Move BuildSignatureFromGlobalConfiguration() to Configuration
Enforce reference lookup test coverage
Enforce checkout test coverage
Fix Checkout behavior when dealing with Head
Teach Checkout() to append to the reflog

When no user information exists in the configuration, a dummy signature
is created to value the reflog entry.
Code style cleanup
Teach UpdateTarget() to append to the reflog
Move Repository.Conflicts to Index.Conflicts
Refactor handling of reference prefixes
Add helper methods to determine reference kind from canonical name
Add extension methods to determine reference kind