tests: use argparse in get-with-headers.py I'm about to add another flag and I don't want to deal with this organic, artisanal argument parser. Differential Revision: https://phab.mercurial-scm.org/D1920

convert: use a collections.deque This function was doing a list.pop(0) on a list whose length was the number of revisions to convert. Popping an early element from long lists is not an efficient operation. collections.deque supports efficient inserts and pops at both ends. So we switch to that data structure. When converting the mozilla-unified repository, which has 445,748 revisions, this change makes the "sorting..." step of `hg convert --sourcesort` significantly faster: before: ~59.2s after: ~1.3s Differential Revision: https://phab.mercurial-scm.org/D1934