Generate patches for a FreeBSD port

Summary:
This command is intended to be used to assist a new port:
prepare your port, use make extract, create a git repository
in the working directory (work/software) and patch the files
as needed, for example until you succeed to build or install.

Then run, the command and it will populate the files/ port
directory with patches files matching your diffs.

Test Plan:
Code tested on Grip and Ysul.
Style checked through expand and Komodo static analysis.

Reviewers: dereckson

Reviewed By: dereckson

Differential Revision: https://devcentral.nasqueron.org/D1144