Introduce repo.Network.FetchHeads
Update libgit2 binaries to 5f9f69d

https://github.com/libgit2/libgit2/compare/586f71e...5f9f69d
Enhance Configuration test coverage

Fix git-tfs/git-tfs#300
Add build.libgit2sharp.x64.cmd
Check all [Flags] enums for overlaps

Closes #311
Initial push implementation
Fix AheadBy/BehindBy behavior for orphaned branch
Teach BranchCollection to disambiguate local and remote branch names