Fix case-correcting completion stomping token on common prefix

The prefix has already been case-corrected at this point and the remaining
completions are for the suffix only.

Fixes #7211

Introduced in
28d67c8f Show completion list on Tab also if a common prefix was inserted

src/reader.cpp

@@ -1812,6 +1812,7 @@ bool reader_data_t::handle_completions(const completion_list_t &comp, size_t tok
 
     if (use_prefix) {
         for (completion_t &c : surviving_completions) {
+            c.flags &= ~COMPLETE_REPLACES_TOKEN;
             wcstring &comp = c.completion;
             comp.erase(comp.begin(), comp.begin() + common_prefix.size());
         }