Don't try to fetch remotes without configured refspecs.

gitup/update.py

@@ -94,6 +94,12 @@ def _fetch_remotes(remotes):
 
     for remote in remotes:
         print(INDENT2, "Fetching", BOLD + remote.name, end="")
+
+        config_attr = "remote.{0}.fetch".format(remote.name)
+        if not _read_config(remote.repo, config_attr):
+            print(":", YELLOW + "skipped:", "no configured refspec.")
+            continue
+
         try:
             results = remote.fetch(progress=_ProgressMonitor())
         except exc.GitCommandError as err: