sparse: move pruning of temporary includes into core
This was our last method on the custom repo type, meaning we could
remove that custom type and inline the 2 lines of code into
reposetup().
As part of the move, instead of wrapping merge.update() from
the sparse extension, we inline the function call. The ported
function now no-ops if sparse isn't enabled, making it safe to
always call.
The call site in update() may not be the most appropriate. But
it matches the previous behavior, which is the safest thing
to do. It can be improved later.
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