Commit: Introduce CommentaryChar and PrettifyMessage in CommitOptions
Drop obsolete members after release v0.18.0
Update libgit2 binaries to 90befde

https://github.com/libgit2/libgit2/compare/2f6f6eb...90befde
Tidy up local cloning test helper method
Test cloning from a new repo in a temporary path.
Release LibGit2Sharp v0.18.1
Introduce RemoteCollection.Rename.
Prevent memory leak on branch removal

Actually, git_branch_delete() no longer frees the reference handle for
the caller. We can now let the handle take care of this when it's
disposed.
Fix xml doc related warnings
Introduce ObjectDatabase.ShortenObjectId()

Fix #677
Teach OdbBackend to expose ExistsPrefix
Make OdbBackend Read[Prefix]() return UnmanagedMemoryStream
Introduce OdbBackend.AllocateAndBuildFrom()
Decorrelate IDisposable implementation from freeing of OdbBackend resources