Teach BranchUpdater to set merge branch directly
Stop sorting collections
Add SkippableTheoryAttribute
Add PathCase and related Repository members
Respect core.ignorecase in BuildRelativePathFrom()

Fixes #214
Remove obsolete Create/Delete methods
Remove obsolete CommitCollection interfaces
Remove obsolete LibGit2Exception
Remove obsolete Configuration members/extensions
Remove obsolete DiffTarget
Remove miscellaneous obsolete members