make new `copy-diffs` work over ssh

move to copy-diffs-rsync
restore add/init mirror remote scripts
restore old copy-diffs behavior
use actual tmp dirs from mktemp in copy-diffs-test
fix sys.path construction in add-mirror-remote
move copy-diffs-test off of cloning git-helpers

travis gives us a “shallow” clone of git-helpers and that was causing
issues with this test, which depends on arbitrary things about the repo
existing.
Merge pull request #49 from ryan-williams/copy-diffs

restore old copy-diffs implementation