Code review comment for lp:~gz/bzr/require_unicode_committer_614593

Implemented those changes, but am now worrying over some new things.

Using per_repository seems wasteful as the guard triggers before the repo actually gets touched, so the 22 variations aren't interestingly different.

Also, CommitBuilder has an existing _validate_unicode_text used with the message and rev props I could have used for the committer check... but it doesn't actually do what it says. Which suggests they also could introduce bad bytestrings to the lower levels.