Tighten a regular expression in repository.create

Summary:
See https://github.com/facebook/phabricator/issues/433. We were missing a "^" here.

This should be moved over to transactions soon and then we can get rid of the duplication. :/

Test Plan: Tried to create a repository with callsign "9X", got a helpful error about "ALL UPPERCASE LETTERS".

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Differential Revision: https://secure.phabricator.com/D7531