opam: provide a dev-repo field to silence warnings

lib/mirage_configure.ml

@@ -67,10 +67,16 @@ let find_git () =
 
 let configure_opam ~name info =
   let open Codegen in
-  let subdir, git_info =
+  let subdir, git_info, dev_repo =
     match find_git () with
-    | Error _ -> None, None
-    | Ok (subdir, branch, git_url) -> subdir, Some (branch, git_url)
+    | Error _ -> None, None, "git+https://example.com/nonexistent"
+    | Ok (subdir, branch, git_url) ->
+      (* TODO is there a library for git urls anywhere? *)
+      let git_url = match Astring.String.cut ~sep:"github.com:" git_url with
+        | Some (_, post) -> "git+https://github.com/" ^ post
+        | _ -> git_url
+      in
+      subdir, Some (branch, git_url), git_url
   in
   let file = opam_path ~name in
   with_output file (fun oc () ->
@@ -81,6 +87,7 @@ let configure_opam ~name info =
       append fmt {|authors: "dummy"|};
       append fmt {|homepage: "dummy"|};
       append fmt {|bug-reports: "dummy"|};
+      append fmt "dev-repo: %S" dev_repo;
       Format.fprintf fmt {|build: [ "sh" "-exc" "|};
       (match subdir with
        | None -> ()
@@ -108,12 +115,7 @@ let configure_opam ~name info =
       (match git_info with
        | None -> ()
        | Some (branch, origin) ->
-         (* TODO is there a library for git urls anywhere? *)
-         let public = match Astring.String.cut ~sep:"github.com:" origin with
-           | Some (_, post) -> "git+https://github.com/" ^ post
-           | _ -> origin
-         in
-         append fmt {|url { src: "%s#%s" }|} public branch);
+         append fmt {|url { src: "%s#%s" }|} origin branch);
       R.ok ())
     "opam file"