This is a convenience class to replace using NSStrings to store and compare SHAs. PBGitSHA has a much faster isEqual: function.
It is <NSCopying> compliant and implements isEqual: and hash so it can be used as a key in dictionaries.
That's why it's there, after all. This also makes the system
more robust and catches some more errors.
The next thing this enables is to also allow right-clicking
on refs in the commit list.
David Catmull