Ticket 47611 - Add script to build patched RPMs
    
    This adds the ability to build patched RPMs, which can be useful
    for using with scanning tools like Coverity.  The script will
    create the source tarball from HEAD of the current branch, but it
    will also add patch files to the specfile if any are found in the
    rpm directory in the source tree.  The intended way of using this
    is as follows:
    
        git diff master workbranch > rpm/workbranch.patch
        make -f rpm.mk patch_rpms
    
    The patch files must end in .patch.  Make targets are added for
    patch_srpms and patch_rpms.
    
    https://fedorahosted.org/389/ticket/47611
    
    Reviewed by: mreynolds