[checkupdates] update submodules according to tag

In case the version information is inside a submodule we need to
checkout the submodule at the version of the tag we test.

Found with org.courville.nova.

Closes: #622

fdroidserver/checkupdates.py

@@ -141,6 +141,7 @@ def check_tags(app, pattern):
     for tag in tags:
         logging.debug("Check tag: '{0}'".format(tag))
         vcs.gotorevision(tag)
+        try_init_submodules(app, last_build, vcs)
 
         if app.UpdateCheckData:
             filecode, codeex, filever, verex = app.UpdateCheckData.split('|')