lib/oe/patch: Make extractPatches() not extract ignored commits

lib/oe/patch: Make extractPatches() not extract ignored commits

If a commit is marked with "%% ignore" it means it is used by devtool to
keep track of changes to the source code that are not the result of
running do_patch(). These changes need to actually be ignored when
extracting the patches as they typically make no sense as actual patches
in a recipe.

This also adds a new test for oe-selftest that verifies that there are
no patches generated from ignored commits.

Signed-off-by: Peter Kjellerstedt <peter.kjellerstedt@axis.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>