ui: make sure buffer is flushed before waiting for user input (issue5587)
Without this patch on Windows 'hg ci -i' hangs waiting for user input
and "examine changes to 'file'? [Ynesfdaq?]" is never displayed (at least
if the diff is sufficiently small). When Ctrl+C is pressed, this prompt
becomes visible, which suggests that the buffer just wasn't flushed.
I've never seen this happening on Linux, but this looks harmless enough
to not platform-gate it.