maintainers/scripts/haskell: Improve commit messages

[maralorn]

This addresses concerns regarding the length of commit messages created by the Haskell maintainer scripts.

I also refactored the update-stackage.sh script a bit.

[maralorn]

Example commit messages look like this: (I don‘t want to disrupt the branch right now by pushing those.)

commit 9a36e70fc5f5ba38cd56bb54930b71618ea31422 (HEAD)
Author: Malte Brandy <malte.brandy@maralorn.de>
Date:   Tue May 4 13:59:31 2021 +0200

    hackage-packages.nix: Regenerate based on current config

    This commit has been generated by maintainers/scripts/haskell/regenerate-hackage-packages.sh

commit ecc970acdbc1662b3b57d6c97f8db11080a0f378
Author: Malte Brandy <malte.brandy@maralorn.de>
Date:   Tue May 4 13:57:49 2021 +0200

    all-cabal-hashes: 2021-05-03T20:39:01Z -> 2021-05-04T11:48:25Z

    This commit has been generated by maintainers/scripts/haskell/update-hackage.sh

commit 35ae9580e37c35d49eaac1807c7fc121dba12043 (HEAD)
Author: Malte Brandy <malte.brandy@maralorn.de>
Date:   Tue May 4 14:18:15 2021 +0200

    Stackage Nightly: 2021-05-01 -> 2021-05-03

    This commit has been generated by maintainers/scripts/haskell/update-stackage.sh

[maralorn]

I hope those commit messages are concise and also understandable for random strangers stumbling over them.

[cdepillabout]

I hope those commit messages are concise and also understandable for random strangers stumbling over them.

I like these new commit messages for exactly these reasons.