can't close *Completions*

[Andrew Yates]

When I use comint-dynamic-complete-filename, the frame is split and a
*Completions* window is opened. At the bottom of *Completions*, it
says "Hit space to flush." Pressing space immediately does close
*Completions*, but if I press any other key before pressing space,
space no longer closes the window. I can close it with kill-buffer or
by switching to that buffer and closing it, but that's inefficient.
Is there a way to make RET close the buffer, just as space does? I'm
using that function for file completion in a terminal, so closing it
with RET makes sense in this case.