cli: have complete_newline() return String instead of mutating

String -> String function is more convenient in one liner, and it's unlikely
we'll use this function in hot loop.

src/cli_util.rs

@@ -1655,9 +1655,8 @@ impl DescriptionArg {
 }
 
 impl From<String> for DescriptionArg {
-    fn from(mut s: String) -> Self {
-        complete_newline(&mut s);
-        DescriptionArg(s)
+    fn from(s: String) -> Self {
+        DescriptionArg(complete_newline(s))
     }
 }
 
@@ -1673,10 +1672,12 @@ impl AsRef<str> for DescriptionArg {
     }
 }
 
-pub fn complete_newline(s: &mut String) {
+pub fn complete_newline(s: impl Into<String>) -> String {
+    let mut s = s.into();
     if !s.is_empty() && !s.ends_with('\n') {
         s.push('\n');
     }
+    s
 }
 
 #[derive(Clone, Debug)]

src/commands/mod.rs

@@ -1751,8 +1751,7 @@ fn edit_description(
         .filter(|line| !line.starts_with("JJ: "))
         .join("\n");
     description.truncate(description.trim_end_matches('\n').len());
-    cli_util::complete_newline(&mut description);
-    Ok(description)
+    Ok(cli_util::complete_newline(description))
 }
 
 fn cmd_describe(