home: markdown formatting for changelogs in git

I discovered `guix pull --news` recently and realised I barely ever keep
up with nixpkgs's change logs, so here's a thing which might help to
change that. It's actually a little more general than I thought it'd
need to be — turns out that people do change old release notes items in
nixpkgs (mostly to update links), so it can format deletions as well.

(some vague inspiration was given by https://github.com/netj/markdown-diff)

It's a little cursed, but then so is using `lowdown` at all I'd say …

1 files changed, 27 insertions, 2 deletions

diff --git a/home/home-minimal.nix b/home/home-minimal.nix
index fa969a6..42f358d 100644
--- a/home/home-minimal.nix
+++ b/home/home-minimal.nix
@@ -65,7 +65,7 @@
         body = "";
       };
       md = ''
-        lowdown -tterm --term-columns=(tput cols) $argv[1] | less
+        lowdown -tterm --term-columns=(tput cols) $argv | less -F
       '';
       mds.body = ''
         if [ "$argv[1]" = "" ]
@@ -83,7 +83,32 @@
                 echo (git -C $git -C .. rev-parse --show-toplevel) (git --git-dir $git remote get-url origin)
             end
         end | fzf --preview 'git -C {1} show --no-show-signature --color=always' | cut -f1 -d" "
-    '';
+      '';
+      # Usage as with `git diff`: show-changelog <old-commit> <new-commit> -- <path/to/changelog>
+      # useful in git post-merge (or similar) hooks. default is to show changes in nixpkgs.
+      show-changelog.body = ''
+        if test (count $argv) = 0
+          set -a argv "HEAD@{1}" -- nixos/doc/manual/release-notes
+        end
+        git diff \
+          -U0 \
+          --word-diff=plain \
+          # format markdown links as atomic changes
+          --word-diff-regex='\[[^]]*\]\([^\)]*\)|[^[:space:]]' \
+          $argv |
+        sed -E '
+          # delete diff noise
+          /^(\+\+\+|diff|@@|index).*$/d
+          # make header per file
+          s|^---.*/([^/]*)$|\n# \1\n|g
+          # do not mark whole-line adds
+          s|^\{\+(.*)\+\}$|\1|g
+          # format word changes
+          s|\{\+(.*)\+\}|**\1**|g
+          s|\[-(.*)-\]|~~\1~~|g
+        ' |
+        md --term-nolinks
+      '';
     };
   };