Merge branch 'jt/submodule-pull-recurse-rebase'

"git pull -recurse-submodules --rebase", when the submodule
repository's history did not have anything common between ours and
the upstream's, failed to execute.  We need to fetch from them to
continue even in such a case.

* jt/submodule-pull-recurse-rebase:
  submodule: do not pass null OID to setup_revisions

submodule.h

@@ -92,7 +92,10 @@ extern int bad_to_remove_submodule(const char *path, unsigned flags);
 
 int add_submodule_odb(const char *path);
 
-/* Checks if there are submodule changes in a..b. */
+/*
+ * Checks if there are submodule changes in a..b. If a is the null OID,
+ * checks b and all its ancestors instead.
+ */
 extern int submodule_touches_in_range(struct object_id *a,
 				      struct object_id *b);
 extern int find_unpushed_submodules(struct oid_array *commits,