Don't keep subcommand stdout around

take() and pass it to BufReader so it gets closed when the reader stops.
Otherwise on an early pager (EPIPE), and thus delta() exit the feeding
subcommand still has an open stdout to write something into and
wait()-ing on it hangs.

src/main.rs

@@ -228,7 +228,7 @@ pub fn run_app(
 
         let cmd_stdout = cmd
             .stdout
-            .as_mut()
+            .take()
             .unwrap_or_else(|| panic!("Failed to open stdout"));
         let cmd_stdout_buf = io::BufReader::new(cmd_stdout);