The name-rev command has an option --undefined (which is not documented in git-name-rev.txt) that doesn't do anything. If you look at builtin-name-rev.c, allow_undefined never gets set to anything but 1, I think. In that case --undefined could be made a dummy option for compatibility and the allow_undefined code could be ripped out.
In the same file, there's also an "else if (always)" clause that, if allow_undefined is indeed always true, never gets run. "git-name-ref --tags --always master" still prints "undefined" despite the --always option. (--always is undocumented as well.)
Perhaps someone more knowledgeable should take a look at this (and maybe write up a test case or two), I don't feel confident submitting patches to this file.
-- Lea