First part of the format-upgrade support in repo

 In order to work across major bumps, we need to add a call to
s6-rc-format-upgrade before s6-rc-update in the s6-rc-set-install
sequence of commands.
 s6-rc-format-upgrade only works when the db is exactly the same
across s6-rc versions. That means we need to have the source version
of the current live set at all times, in order to compile it with
the new s6-rc version before upgrading and then updating.
 So:
 1. we take a snapshot of the source version of a set every time
we commit.
 2. we refuse to install if we don't have the snapshot.
 3. we add s6-rc-format-upgrade to the processing chain
 4. for cleanup: after installing, we delete all the snapshots of
the set except the one that corresponds to the newly
installed live set.

 This commit implements 1: taking the snapshot. It's a regular
s6-rc-set-copy, using an internal name. We refactor
s6-rc-set-copy and s6-rc-set-delete to have primitives we can
use internally.

 Wait for later commits before using repo, else you'll fill
your repository with snapshots that won't get cleaned up.

 TBD: how to store the s6-rc version in the compiled db. We
probably need to add a small file in there.

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