Call dmon_unwatch when changing project

Fix a conspicuous omission to call the dmon_unwatch function
when changing project directory.

This uncovered a bug or a quirk of the dmon library where the watch_ids
can change as a result of calling dmon_unwatch because they are just
indexes on a contiguous array. Use a workaround to always unwatch the
first valid watch_id N times.

data/core/commands/core.lua

@@ -164,7 +164,10 @@ command.add(nil, {
         core.error("Cannot open folder %q", text)
         return
       end
-      core.confirm_close_docs(core.docs, core.open_folder_project, text)
+      core.confirm_close_docs(core.docs, function(dirpath)
+        core.close_current_project()
+        core.open_folder_project(dirpath)
+      end, text)
     end, suggest_directory)
   end,