core/packs/: Enable undo-tree

Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>
core/packs/pack-git: Add magithub

Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>
core/packs/: Add a paste pack

This includes refheap only for now.

Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>