Demonstrate target promotion bug (ocaml#5292)

Signed-off-by: Andrey Mokhov <[email protected]>

test/blackbox-tests/test-cases/promote/promote-only-when-needed.t

@@ -0,0 +1,23 @@
+Test that targets aren't re-promoted if they are up to date.
+
+  $ echo "(lang dune 3.0)" > dune-project
+  $ cat >dune <<EOF
+  > (rule
+  >  (targets promoted)
+  >  (mode promote)
+  >  (action (with-stdout-to promoted (echo "Hello, world!"))))
+  > EOF
+
+  $ dune build promoted --verbose 2>&1 | grep "Promoting"
+  Promoting "_build/default/promoted" to "promoted"
+  $ cat promoted
+  Hello, world!
+
+Bug: Dune promotes the file again even though its unchanged.
+
+# CR-someday amokhov: Fix this.
+
+  $ dune build promoted --verbose 2>&1 | grep "Promoting"
+  Promoting "_build/default/promoted" to "promoted"
+  $ cat promoted
+  Hello, world!