Keep a log over time with the config commits

m/common/rev.nix

@@ -0,0 +1,18 @@
+{ theFlake, ... }:
+
+let
+  rev = if theFlake ? rev then theFlake.rev
+    else throw ("Refusing to build from a dirty Git tree!");
+in {
+  # Save the commit of the config in /etc/configrev
+  environment.etc.configrev.text = rev + "\n";
+
+  # Keep a log with the config over time
+  system.activationScripts.configRevLog.text = ''
+    BOOTED=$(cat /run/booted-system/etc/configrev 2>/dev/null || echo unknown)
+    CURRENT=$(cat /run/current-system/etc/configrev 2>/dev/null || echo unknown)
+    NEXT=${rev}
+    DATENOW=$(date --iso-8601=seconds)
+    echo "$DATENOW booted=$BOOTED current=$CURRENT next=$NEXT" >> /var/configrev.log
+  '';
+}