Support running script against local changes

Some projects do not strictly enforce requirements changes and
developers in those project need a way to see how far away they
are from the global requirements. So we need a way to run this
script to figure out which of those items in global requirements
they would like to update in their projects.

Adding a --local optional flag and defaulting the branch to 'master'
seems to support both existing usecases in gate and the local
use case as detailed above

Change-Id: I76fcea1e2965795ebb99f2b7d649cf9a53908f09
Merge "Support running script against local changes"