I'm going to redirect progress message to stderr, but the current Ui API
makes it less clear whether the underlying streams of .write*()/.flush()/
.output_guard() are related or not.
Since stdout is wrapped with LineWriter, we need to explicitly flush the
output. Otherwise the snapshot progress would persist until newline character
is printed.
It might also be better to send progress to stderr?
I'm going to add $COLUMNS override, and it should work even if ioctl() on tty
failed. This means that the return type has to be (Option<u16>, Option<u16>).
Since we don't use the row count, I decided to drop it.
I ran an upgraded Clippy on the codebase. All the changes seem to be
about using variables directly in format strings instead of passing
them as separate arguments.
This was actually what I meant in my code-review comment but I used
the wrong word ("clip") :) I wasn't going to bother changing it, but
Clippy nightly just started warning about it.
