add dynamic node attribute for last apply

so we can check if something has changed in the repo since the last
apply

hooks/deploy_commit_hash_to_node.py

@@ -0,0 +1,5 @@
+def node_apply_end(repo, node, duration, interactive, result, **kwargs):
+    if not node.os in node.OS_FAMILY_UNIX:
+        return
+
+    node.run(f'echo "{repo.revision}" > /var/lib/bundlewrap/last_apply_commit_id')

nodes/attributes.py

@@ -0,0 +1,24 @@
+from bundlewrap.utils.ui import io
+from bundlewrap.utils.scm import get_rev
+from bundlewrap.utils.text import red, bold
+
+@node_attribute
+def needs_apply(node):
+    if node.dummy:
+        return False
+
+    if node.os not in node.OS_FAMILY_UNIX:
+        return True
+
+    try:
+        applied = node.run(
+            'cat /var/lib/bundlewrap/last_apply_commit_id',
+            may_fail=True,
+        ).stdout.decode().strip()
+
+        if not applied or applied != get_rev():
+            return True
+    except Exception as e:
+        io.stderr(f'{red("!!!")}  {bold(node.name)}  {e!r}')
+
+    return False