ui: ensure progress completion output is flushed

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?

src/progress.rs

@@ -37,6 +37,7 @@ impl Progress {
 
         if progress.overall == 1.0 {
             write!(ui, "\r{}", Clear(ClearType::CurrentLine))?;
+            ui.flush()?;
             return Ok(());
         }
 

src/ui.rs

@@ -331,5 +331,6 @@ pub struct OutputGuard {
 impl Drop for OutputGuard {
     fn drop(&mut self) {
         _ = self.output.write_all(self.text.as_bytes());
+        _ = self.output.flush();
     }
 }