api: allow filtering patches and covers by msgid

api: allow filtering patches and covers by msgid

In the process of fixing the previous bug, I realised that:

 a) /api/patches/msgid is a perfectly reasonable thing to attempt
 b) We have no way of finding a patch by message id in the API

We can't actualy make /api/patches/msgid work because it may not
be unique, but we can add a filter.

I'm shoehorning this into stable/2.2, even though it's technically
an API change: it's minor, not incompatible and in hindsight a
glaring hole.

Cc: Michael Ellerman <mpe@ellerman.id.au>
Tested-by: Jeremy Kerr <jk@ozlabs.org>
Reviewed-by: Andrew Donnellan <ajd@linux.ibm.com>
Reviewed-by: Stephen Finucane <stephen@that.guru>
Signed-off-by: Daniel Axtens <dja@axtens.net>