Add an assertion on commit message.

This is what's likely to change while I'm working on this branch. This
is going to be easier to debug than a commit sha that changes.
Pass a Tree instead of a sha in the LogEntry.
Create commits with LibGit2Sharp.

This breaks all the clone tests because the commit shas change. The
commit shas change because libgit2sharp changes "\r\n" to "\n".
Make sure the tree is right, too.
Extract the tree & commit assertions.
Ignore line endings in commit messages.
Merge better CloneTests.
Update commit shas.
Add this to the release notes.
Merge pull request #534 from spraints/commit-with-lg2s

Use libgit2sharp to commit
Add mention for #534 in release notes