commenter: snapshot-diff

[bergmark]

Example output: #6217 (comment)

From CURATORS.md:

Diffing snapshots / Inspecting changes

To diff existing snapshots, or to evaluate changes before they end up
in a snapshot you can run:

commenter diff-snapshot <old-snapshot.yaml> <new-snapshot.yaml>

Existing snapshots can be retrieved from https://github.com/commercialhaskell/stackage-snapshots. Preliminary snapshots can be generated by running relevant parts of automated/build.sh, at the time of writing:

TARGET=nightly-2021-01-14 \ # the date doesn't matter
  curator update && \
  curator constraints --target $TARGET && \
  curator snapshot-incomplete --target $TARGET && \
  curator snapshot

[bergmark]

In hindsight, there is definitely code for this somewhere already, probably in stackage-server? Anyway, parsing snapshot files is very simple.