Improve the diff-clean plugin

Following the proposal from @mattia in !123 (merged):

The diff-clean plugin will be of a little use since it only checks for format 1.0 source packages, those being rare. That could be enhanced by adding information about patches for other the quilt format.

We could report some info, such as:

the patch name
the patch message
impacted files
a diifstat

The plugin could also be merged to the source-format plugin as it collides with its scope. Note that we can still keep two QA reports by using two tags inside the plugin.

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information