Consider adding special case of updating the 'AllProposals' snapshot #40
Description
The snapshot command can use an existing snapshot as its source, with any additional files such as READMEs being copied to the new snapshot. This is useful when the extraction logic of the tool changes, to generate the updated expected results.
One special case is the 'AllProposals' snapshot that includes a snapshot of the swift-evolution proposals.
It likely would be useful to have a special case that gets all current proposals from the repository and copies any ancillary files from the original snapshot. Potentially, this special case could automatically replace the existing snapshot in place.