vcs: fix get_changesets to .reverse() list of revision hashes in place instead of using reversed()

This is slightly more performant, and will also make CollectionGenerator work
with reverse=True. Before, CollectionGenerator was used on get_changesets, but
never with reverse option. Trying to use reverse, it would fail when applying
len():

TypeError: object of type 'listreverseiterator' has no len()