Don't complete files if no completion can be found

This essentially breaks the "--no-files" flag to `complete`.

Some commands simply do _not_ take files.

Fixes #112.

src/complete.cpp

@@ -1451,10 +1451,6 @@ void complete(const wcstring &cmd_with_subcmds, std::vector<completion_t> *out_c
                         }
                     }
 
-                    // If we have found no command specific completions at all, fall back to using
-                    // file completions.
-                    if (completer.empty()) do_file = true;
-
                     // Hack. If we're cd, handle it specially (issue #1059, others).
                     handle_as_special_cd = (current_command_unescape == L"cd");