This patch also adds new check-code.py pattern to detect invalid usage
of "mercurial@selenic.com".

Change for test-convert-tla.t is tested, but similar change for almost
same test-convert-baz.t isn't yet tested actually, because I couldn't
find out the way to get "GNU Arch baz client".

AFAIK, buildbot skips test-convert-baz.t, too. Does anybody have
appropriate environment for testing?