If cover letter looked like a patch (i.e. had the diff markers) it was
parsed as such, resulting in the diff being extracted out from the
contents and stored as oddly numbered patch 0.

Let's store the raw payload and do not create patches out of cover
letters.

This is now tested by adding some diff excerpt to one of the test
fixtures and adjusting the asserts on cover_letter and raw_cover_letter
accordingly.

Signed-off-by: Arkadiusz Hiler <arkadiusz.hiler@intel.com>