Should forbid committing a revision with an invalid committer

[Max Bowsher]

I've just found in an UDD branch a commit where the committer string contains a linebreak. This is a problem because it's impossible to create a testament for such a revision, because of format restrictions validated in Testament.__init__.

However, "bzr whoami" is willing to set up such a committer, and "bzr commit" is willing to commit it.

Given such a construct is invalid in some places in the codebase, it should be checked and caught during whoami and commit.