Handle prune-older pruning all; improve tests; test --pretend

[Rob Browning]

Fix some prune older issues (overflow handling, pruning of all saves,
etc.), and add tests, e.g. for invalid invocations, and --pretend.

Also document the fact that right now prune-older only apples to the
last section of commits on the branch that have non-decreasing
timestamps.

Pushed to main.

--
Rob Browning
rlb @defaultvalue.org and @debian.org
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