Script to generate merge summary

[newhoggy]

A merge summary is useful for quickly query for data to add to https://github.com/input-output-hk/cardano-updates

Example output:

$ ./scripts/gen-merge-summary.sh 2023-02-07
- [workbench:  drop cabalWrapped](https://github.com/input-output-hk/cardano-node/pull/4873 from input-output-hk/wb-drop-cabalWrapped) (Kosyrev Serge)
- [workbench:  `make shell-nix` use Nix-built binaries](https://github.com/input-output-hk/cardano-node/pull/4872 from input-output-hk/wb-shell-nix) (Kosyrev Serge)
- [Revert #4855](https://github.com/input-output-hk/cardano-node/pull/4870 from input-output-hk/jordan/remove-4855) (Jordan Millar)
- [Remove cardano-cli's dependency on cardano-node](https://github.com/input-output-hk/cardano-node/pull/4855 from input-output-hk/jordan/remove-cli-node-dependency) (Jordan Millar)

[angerman]

@newhoggy can we add (to the PR description; and the script) the motivation for this? Why are we doing this, what do we want to get out of this? I fear without that motivation, someone else looking at it will not be a bit lost.