Move `RewriteHistory` to the `Repository` class
Avoid potential null dereference
Rename CommitHeader to CommitRewriteInfo
Add optional parameters to CommitRewriteInfo.From
Back up refs to customizable namespace

Dealing with existing backed-up refs is left
as an exercise to the reader
Add parent rewriting
Use a better test failure message
Move reachability test to IEnumerable<Reference> extension
Clean up test
Add XML-doc comments
Remove ability to change commit-message encoding
Filter-branch: allow prevention of backing up refs