Fix function definitions potentially leaking as completions

src/complete.cpp

@@ -655,6 +655,15 @@ static wcstring complete_function_desc(const wcstring &fn) {
     bool has_description = function_get_desc(fn, result);
     if (!has_description) {
         function_get_definition(fn, result);
+        // Consider complete -a '(complete -C "prefix")':
+        // If some functions whose name starts with prefix and whose
+        // definition includes a line that starts with prefix, this line
+        // would be suggested as completion.
+        // Replace newlines by spaces to avoid these excess lines.
+        // The completion description will be shown in one line regardless.
+        for (wchar_t &c : result) {
+            if (c == L'\n') c = L' ';
+        }
     }
     return result;
 }