describe: add --lang back into dune-describe_opam_files for backcompat

Signed-off-by: Ali Caglayan <[email protected]>

Author: Alizter

bin/describe/describe_opam_files.ml

@@ -3,7 +3,16 @@ open Stdune
 
 let term =
   let+ common = Common.term
-  and+ format = Describe_format.arg in
+  and+ format = Describe_format.arg
+  and+ _ =
+    Arg.(
+      value
+      & opt (some string) None
+      & info [ "lang" ] ~docv:"VERSION"
+          ~doc:
+            "This argument has no effect and is deprecated. It exists solely \
+             for backwards compatibility.")
+  in
   let config = Common.init common in
   Scheduler.go ~common ~config @@ fun () ->
   Build_system.run_exn @@ fun () ->

test/blackbox-tests/test-cases/describe/describe.t

@@ -1343,7 +1343,7 @@ Test errors
 opam file listing
 -----------------
 
-  $ dune describe opam-files | dune_cmd expand_lines
+  $ dune describe opam-files --lang 0.1 | dune_cmd expand_lines
   ((foo.opam
     "# This file is generated by dune, edit dune-project instead
   opam-version: \"2.0\"