Clean-up trailing whitespaces

For the psychopath that lies deep down in each one of us.
Move Stage()/Unstage() tests to their own files
Update libgit2 binaries to 6950dca

https://github.com/libgit2/libgit2/compare/ad00376...6950dca
Stop sorting collections
Add SkippableTheoryAttribute
Add PathCase and related Repository members
Respect core.ignorecase in BuildRelativePathFrom()

Fixes #214