Change sq packet split to write to stdout by default.

- Change `sq packet split` to not require `output` or `prefix`, but
    to write to `stdout` by default.

  - This is closer to the behavior of other commands.

NEWS

@@ -105,6 +105,7 @@
      those that emit signed and or encrypted messages.
    - The command `sq toolbox extract-cert` has been removed in favor
      of `sq key delete` and `sq key subkey delete`.
+   - The command `sq packet split` now writes to stdout by default.
 
 * Changes in 0.39.0
 ** Notable changes

src/cli/packet.rs

@@ -5,7 +5,7 @@ use std::{
     path::PathBuf,
 };
 
-use clap::{ArgGroup, Args, Parser, Subcommand};
+use clap::{Args, Parser, Subcommand};
 
 use crate::cli::examples::*;
 use crate::cli::types::ArmorKind;
@@ -263,7 +263,6 @@ The converse operation is `sq packet join`.
 ",
     after_help = SPLIT_EXAMPLES,
 )]
-#[clap(group(ArgGroup::new("sink").args(&["output", "prefix"]).required(true)))]
 pub struct SplitCommand {
     #[clap(
         default_value_t = FileOrStdin::default(),

src/commands/packet.rs

@@ -134,8 +134,8 @@ pub fn split(sq: Sq, c: SplitCommand) -> Result<()>
     let mut sink = match c.prefix {
         Some(p) => Err(p),
         None => Ok(
-            c.output.as_ref()
-                .expect("either prefix or output must be given")
+            c.output
+                .unwrap_or_default()
                 .create_pgp_safe(&sq, true, Kind::SecretKey)?),
     };