Configurable pristine checksum kind in a working copy
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The implementation on the branch allows creating working copies that use a
checksum kind other than SHA-1.

The checksum kind is persisted in the settings table.  Upgraded working copies
of the older formats will have SHA-1 recorded as their pristine checksum kind
and will continue to use it for compatibility.  Newly created working copies
of the latest format (with --compatible-version=1.15 or --store-pristine=no),
as currently implemented, will use the new pristine checksum kind.

Currently, as a proof-of-concept, the branch uses salted SHA-1 as the new
pristine checksum kind.  For the production-ready state, we could support
using multiple new checksum types such as SHA-256.  That could be useful
for future compatibility, because if we encounter any issues with one
checksum kind, we could then switch to a different kind without having
to change the working copy format.