rpm: implement file path filters in patch export

rpm: implement file path filters in patch export

Implements a filter option allows one to filter out changes to certain
files/paths in the patch-generation. A commit is totally ignored if all
files would be filtered out. The path filter is given as a Python
regexp.

This option is useful for example in filtering out the changes to
packaging files when maintaining packaging and sources in the same
branch.

Signed-off-by: Markus Lehtonen <markus.lehtonen@linux.intel.com>