import-srpm: support for patch import

import-srpm: support for patch import

Adds a new commandline option '--patch-import' for importing patches
into the source tree in packaging branch. When enabled, gbp applies and
commits all patches (not marked for manual maintenance) into the
packaging branch. If this succeeds, it also removes all imported patch
files from the packaging directory and the spec file.

Currently only supported for non-orphan-packaging.

This setting is true by default which should make more sense as the
developer wants to do code development in the packaging branch (if
he/she selects to use non-orphan packaging).

However, patch-import is force-disabled if in bare git repository. We
cannot support this, currently.

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