Enforce single-line subjects

We already enforce a maximum (60 character) length on commit message
subject lines to encourage succint messages. However, if a newline isn't
inserted below the subject and more text is present, git interprets this
as a continuation.

Change-Id: I4554ebfa2ca0e421054cfa0d11c313813f3e341d
Reviewed-on: https://gerrit.causes.com/23510
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Greg Hurrell <greg@causes.com>