Add git-exclude

A Git commit that easily excludes files by adding them to
`.git/info/exclude`.

modules/home-manager/cli/git.nix

@@ -169,6 +169,7 @@ in
 
     home = {
       packages = with pkgs; [
+        git-exclude
         git-extras
         git-graph
         git-trim

packages/default.nix

@@ -14,6 +14,7 @@ in
   dev-commit = callPackage ./dev-commit.nix { };
   displayselect = callPackage ./displayselect { };
   get-tags = callPackage ./get-tags.nix { };
+  git-exclude = callPackage ./git-exclude.nix { };
   git-graph = callPackage ./git-graph.nix { };
   mounter = callPackage ./mounter.nix { };
   move-firefox-screenshots = callPackage ./move-firefox-screenshots.nix { };

packages/git-exclude.nix

@@ -0,0 +1,24 @@
+{ pkgs }:
+
+pkgs.writeShellApplication {
+  name = "git-exclude";
+
+  runtimeInputs = with pkgs; [ git ];
+
+  text = ''
+    # Based on https://github.com/jfly/snow/blob/fb727b39736666f54908f4dc7c83dd6284607c5a/machines/pattern/homies/bin/git-exclude.
+
+    root=$(git rev-parse --show-toplevel)
+    rel=$(realpath --relative-to "$root" .)
+
+    # For some reason, ignore rules like "./.envrc" don't seem to work, but
+    # "/.envrc" does. :shrug:
+    if [ "$rel" = "." ]; then
+      rel=""
+    fi
+
+    for item in "$@"; do
+      echo "''${rel}/$item" >> "$root/.git/info/exclude"
+    done
+  '';
+}