Use LibGit2Sharp.Mode in place of strings.
Split IGitTreeBuilder.

IGitTreeModifier: TfsChangeset only needs the Add and Remove methods. So
those methods are part of IGitTreeModifier.

IGitTreeBuilder: GitTfsRemote needs to pass a IGitTreeModifier to
ITfsChangeset, and call GetTree() too.
:burn: GitIndexInfo
:burn: HashAndInsertObject.
Remove temporary index work.

This dropped the time to run unit tests on my machine from ~8.3s to ~6.6s!
More obsolete code.