pq: rewrite patch export functionality

Use our own function for constructing the patch files instead of using
the format-patch command of git. This way, we get the desired output
format directly, without the need for the error-prone "format-patch,
parse patch files, mangle and re-write patch files" cycle.

Signed-off-by: Markus Lehtonen <markus.lehtonen@linux.intel.com>
GitRepository: drop unused format_patch method

Signed-off-by: Markus Lehtonen <markus.lehtonen@linux.intel.com>
pq.format_patch: support file path filtering

Implements a filter option that allows filtering 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.

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