Merge branch 'ks/combine-diff'
Teach combine-diff to honour the path-output-order imposed by
diffcore-order, and optimize how matching paths are found in
the N-way diffs made with parents.
* ks/combine-diff:
tests: add checking that combine-diff emits only correct paths
combine-diff: simplify intersect_paths() further
combine-diff: combine_diff_path.len is not needed anymore
combine-diff: optimize combine_diff_path sets intersection
diff test: add tests for combine-diff with orderfile
diffcore-order: export generic ordering interface