hg: always show and run Mercurial hooks in alphabetical order (Issue #246)

Mercurial will generally run hooks in the order they are found in the
configuration. For entries found in the database, there is no such order.
Instead, always use alphabetical order for these.

Since we now want to order things explicitly in the db query, we want an index
with a composite key. We do that even though we don't really need it for the
few entries in this table, and even though it might/could use the same index as
the existing unique constraint. This composite UniqueConstraint was added in
b9f4b444a172 where it replaced a wrong UniqueConstraint that could/should have
been removed in c25191aadf92. Fix that while touching this area and running a
migration script.