patch: Single out the commit message

patch: Single out the commit message

All 'Comments' are stored the same way in the db, but I believe it's
worth making the distinction between introducing what the patch does and
eventual review comments.

v3: Fix docstrings
v2: Use two new Patch methods to retrieve the commit message and the
    other comments (called answers here) (Jeremy Kerr)

Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Stephen Finucane <stephen.finucane@intel.com>