Fix some compilation warnings
Enhance checkout test coverage
Introduce OrphanedHeadException
Update libgit2 binaries to 586f71e

https://github.com/libgit2/libgit2/compare/7dfc5c3...586f71e
Make Checkout raise proper MergeConflictException
Enhance Index.RetrieveStatus() test coverage

Partially fix git-tfs/git-tfs#231
Tidy up OdbBackendFixture code
Ensure Checkout() doesn't mess with binary files

Fix #302