generic: add "AffectedFiles" field to Changelist and use it to determine category

[stamblerre]

For changelists without the "pkg/path: message" format of description, it is difficult to make a categorization.
We should add the list of files in the changelist to the generic.Changelist struct, and then use that to extract a category if none exists. The best way to do that would probably be to count directories and choose the most common one or the most common parent.

[stamblerre]

Done in 5afd537.