Fix TrackedBranch property on DetachedHead branches
Align IRepository with public Repository members
Convert repo.Checkout() overloads into extensions methods
Introduce Repository.Info.CurrentOperation
Introduce repo.Index.IsFullyMerged
Introduce IndexEntry.StageLevel
Fix IndexEntry IEquatable implementation
Minor code refactoring
Fix some issues pinpointed by Code Analysis
Ensure Diff.Compare() cannot apply on a bare repository
Enhance Index.Unstage test coverage
Make Index.Unstage() work against an orphaned Head

Fix #257
Enhance Index.Unstage() code coverage
Index.Unstage should accept absolute file paths
Make Diff.Compare() able to compare a null Tree against the Index and Workdir
Enhance Diff.Compare() test coverage
Simplify Index.Unstage() implementation
Enhance Index.[Stage|Unstage]() test coverage
Adds Reset(ResetOptions resetOptions, Commit commit) method in Repository
Adds Reset(Commit commit, IEnumerable<string> paths = null) to Repository