feat(ops/deploy-whitby): Add the start of a script to deploy whitby

Add the beginnings of an auto-deploy script for whitby, intended to
be (eventually) suitable for running automatically in a systemd timer.
The current iteration of the script doesn't actually do any deploying,
but instead takes as an argument a revision, creates a new git worktree
in /tmp with that revision checked out, runs a nix-diff of whitby's
system derivation in the running system and at that closure, puts an
html-rendered version of that diff in the public directory used by
deploy.tvl.fyi, and finally sends a message to IRC via irccat with a
link to that HTML page.

Refs: b/110
Change-Id: Id40525567f8845590c909568befd8d00c07a481c
Reviewed-on: https://cl.tvl.fyi/c/depot/+/3145
Tested-by: BuildkiteCI
Reviewed-by: tazjin <mail@tazj.in>
Reviewed-by: kn <klemens@posteo.de>

ops/deploy-whitby/deploy-whitby.sh

@@ -0,0 +1,45 @@
+#!/usr/bin/env bash
+set -Ceuo pipefail
+
+HTML_ROOT="${HTML_ROOT:-/var/html/deploys.tvl.fyi}"
+URL_BASE="${URL_BASE:-https://deploys.tvl.fyi/diff}"
+IRCCAT_PORT="${IRCCAT_PORT:-4722}"
+
+drv_hash() {
+    basename "$1" | sed 's/-.*//'
+}
+
+new_rev="$1"
+
+if [ -z "$new_rev" ]; then
+    >&2 echo "Usage: $0 <new_rev>"
+    exit 1
+fi
+
+if [ -d "/tmp/deploy.worktree" ]; then
+    >&2 echo "/tmp/deploy.worktree exists - exiting in case another deploy is currently running"
+    exit 1
+fi
+
+worktree_dir=/tmp/worktree_dir
+
+cleanup() {
+    rm -rf "$worktree_dir"
+}
+trap cleanup EXIT
+
+git clone https://cl.tvl.fyi/depot "$worktree_dir" --reference /depot
+git -C "$worktree_dir" checkout "$new_rev"
+
+current=$(nix show-derivation /run/current-system | jq -r 'keys | .[0]')
+new=$(nix-instantiate -A ops.nixos.whitbySystem "$worktree_dir")
+
+diff_filename="$(drv_hash "$current")..$(drv_hash "$new")"
+nix-diff "$current" "$new" --color always \
+    | ansi2html \
+    >| "$HTML_ROOT/diff/$diff_filename"
+
+echo "#tvl whitby is being deployed! system diff: $URL_BASE/$diff_filename" \
+    | nc -w 5 -N localhost "$IRCCAT_PORT"
+
+# TODO(grfn): Actually do the deploy