py3: make hgext/fetch.py use absolute_import

tests: test-check-py3-compat.t output updated The test output was not updated as the lower section of the test updates with python3.5, so it might be the case that people have updated the modules but the test was left as it was. So this patch updates the test output.

revset: factor out public optimize() function from recursion New optimize() hides internal arguments and return values. This makes it easy to add more parameters and return values to _optimize().