update command: produce concise summary of change

[alexeagle]

When automating renovate/dependabot-style freshness PRs, they end up in the changelog of whatever project uses multitool and that makes it hard to parse.

For example aspect-build/rules_lint#343 will end up with entries like the two "Changes from pull-request action"

Perhaps if the update command produces a short description, those automation tools can wire it up to the PR title.