Requirements check should tolerate existing cruft

* .../slave_scripts/project-requirements-change.py: When parsing
requirements files, only perform style checks like missing final
newline or duplicate entries on the new/proposed version of a file
and ignore them on the original version of the file in the
repository. This makes it possible to actually correct errors on
imported requirements files, even with the job running this script
already voting on the correcting change. Also annotate this with
more commentary so people have some hope of understanding what the
heck it's doing.

Change-Id: I8ea2800ecc8eb24937e69a573e925b4daf64ff3f
Closes-Bug: #1300843
Merge "Requirements check should tolerate existing cruft"