Improve patch detection process

[threexc]

Right now, patches submitted for the meta-python layer are detected using git log filters for changes to recipe files (i.e. ".bb" and ".inc"), but this limits a more complete analysis of what has been submitted, and it isn't portable to other layers as-is because of how it relies on "python" to be in the subject line. There should be a more sophisticated way to collect the relevant data to determine what to build.