uniquify selection contents before generating regex for '*'

Avoid generating regex with the same alternative repeated multiple
times.

src/normal.cc

@@ -983,7 +983,7 @@ void use_selection_as_search_pattern(Context& context, NormalParams params)
                       smart and is_bow(buffer, beg) ? "\\b" : "",
                       escape(buffer.string(beg, end), "^$\\.*+?()[]{}|", '\\'),
                       smart and is_eow(buffer, end) ? "\\b" : "");
-    });
+    }) | gather<HashSet>();
     String pattern = join(patterns, '|', false);
 
     const char reg = to_lower(params.reg ? params.reg : '/');