Add update script for heartwood version and flake

Use this script to update tests/support/heartwood-version and keep the
flake in sync with it.

Alternatively, run the script as

    scripts/update-flake $(cat tests/support/heartwood-version)

to bring the flake back into sync.

This also updates the flake.lock and the updates the state of the npm
dependencies when run from a nix-enabled system.

scripts/update-flake

@@ -0,0 +1,75 @@
+#!/bin/sh
+
+set -e
+
+usage() {
+    printf \
+        'Update flake and heartwood-version to the requested heartwood version.
+When called without arguments, reads HASH from tests/support/hearwood-version.
+
+USAGE:
+  update-flake [-h] [PATH|HASH]
+
+OPTIONS:
+  PATH\t\tPath to heartwood git repo; uses current HEAD.
+  HASH\t\tUpdate heartwood version to HASH.
+  -h --help\tPrint this help.\n'
+}
+
+while [ $# -ne 0 ]; do
+    case $1 in
+        --help | -h)
+            usage >&2
+            exit
+            ;;
+        */*)
+            hash=$(git -C "$1" rev-parse --verify HEAD)
+            shift
+            ;;
+        *)
+            hash=$1
+            shift
+            ;;
+    esac
+done
+
+# Heartwood repo id
+rid=z3gqcJUoA1n9HaHKufZs5FCSGazv5
+
+# Some root detection guess work...
+root=$(git rev-parse --show-toplevel 2>/dev/null || echo "$PWD")
+if [ ! -e "$root/tests/support/heartwood-version" ]; then
+    root=$(cd .. ; pwd)
+fi
+
+if [ -z "$hash" ]; then
+    # No hash given, take from heartwood-version
+    if [ -e "$root/tests/support/heartwood-version" ]; then
+        hash=$(cat "$root/tests/support/heartwood-version")
+        [ -n "$hash" ] || {
+            printf "Error reading heartwood-version\n" >&2
+            exit 1
+        }
+    fi
+else
+    printf '%s\n' "$hash" >$root/tests/support/heartwood-version
+fi
+
+if [ -e "$root/flake.nix" ]; then
+    sed -E -e '/url.*git\+https.*'"$rid"'/s/(rev=)[0-9a-f]{40,40}/\1'"$hash"'/' "$root/flake.nix" >$root/flake.nix.tmp
+    mv "$root/flake.nix.tmp" "$root/flake.nix"
+
+    # If running on a nix-enabled system, also update npmDepsHash.
+    if type nix >/dev/null 2>&1; then
+        npmDepsHash=$(nix --experimental-features 'nix-command flakes' \
+            run --inputs-from "$root" nixpkgs#prefetch-npm-deps \
+            -- "$root/package-lock.json")
+        sed -E -e '/npmDepsHash/s/".*"/"'"$npmDepsHash"'"/' "$root/flake.nix" >$root/flake.nix.tmp
+        mv "$root/flake.nix.tmp" "$root/flake.nix"
+
+        nix --experimental-features 'nix-command flakes' flake update "$root"
+    else
+        printf >&2 'Node dependencies may be out of date!
+Re-run on a nix-enabled system to update npmDepsHash.\n'
+    fi
+fi