chore(update_appup): do not force appup render if contents are the same

To avoid losing comments and/or manual indentation in appup files that
are already up to date, we now check whether the contents have the
exact same terms as those we are about to write to an existint .appup
file.

scripts/update_appup.escript

@@ -368,7 +368,12 @@ update_appups(Changes) ->
 do_update_appup(App, Upgrade, Downgrade, OldUpgrade, OldDowngrade) ->
     case locate(src, App, ".appup.src") of
         {ok, AppupFile} ->
-            render_appfile(AppupFile, Upgrade, Downgrade);
+            case contains_contents(AppupFile, Upgrade, Downgrade) of
+                true ->
+                    ok;
+                false ->
+                    render_appfile(AppupFile, Upgrade, Downgrade)
+            end;
         undefined ->
             case create_stub(App) of
                 {ok, AppupFile} ->
@@ -408,6 +413,17 @@ create_stub(App) ->
             false
     end.
 
+%% we check whether the destination file already has the contents we
+%% want to write to avoid writing and losing indentation and comments.
+contains_contents(File, Upgrade, Downgrade) ->
+    %% the file may contain the VSN variable, so it's a script
+    case file:script(File, [{'VSN', 'VSN'}]) of
+        {ok, {_, Upgrade, Downgrade}} ->
+            true;
+        _ ->
+            false
+    end.
+
 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
 %% application and release indexing
 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%