add by-hash sources.list option and document all of by-hash

This changes the semantics of the option (which is renamed too) to be a
yes/no value with the special additional value "force" as this allows
by-hash to be disabled even if the repository indicates it would be
supported and is more in line with our other yes/no options like pdiff
which disable themselves if no support can be detected.

The feature wasn't documented so far and hasn't reached a (un)stable
release yet, so changing it without trying too hard to keep
compatibility seems okay.

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