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