Add git-exclude

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

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
+  '';
+}