Make cmdpy_completer_helper return a gdbpy_ref<> directly rather than
building a gdbpy_ref<>, releasing it, and then having a new
gdbpy_ref<> created to hold the result.
gdb/ChangeLog:
* python/py-cmd.c (cmdpy_completer_helper): Return gdbpy_ref<>.
(cmdpy_completer_handle_brkchars): Adjust.
(cmdpy_completer): Adjust.