Trim completions and ensure final newline (#609)

Trim whitespace at beginning and end of generated completions.
Additionally, since some editors will automatically insert a final
newline into text files, make sure a final newline is present.

completions/just.elvish

@@ -1,4 +1,3 @@
-
 edit:completion:arg-completer[just] = [@words]{
     fn spaces [n]{
         repeat $n ' ' | joins ''

completions/just.powershell

@@ -1,4 +1,3 @@
-
 using namespace System.Management.Automation
 using namespace System.Management.Automation.Language
 

src/common.rs

@@ -6,7 +6,7 @@ pub(crate) use std::{
   env,
   fmt::{self, Debug, Display, Formatter},
   fs,
-  io::{self, Write},
+  io::{self, Cursor, Write},
   iter::{self, FromIterator},
   ops::{Index, Range, RangeInclusive},
   path::{Path, PathBuf},

src/config.rs

@@ -464,7 +464,12 @@ impl Config {
       .parse::<clap::Shell>()
       .expect("Invalid value for clap::Shell");
 
-    Self::app().gen_completions_to(env!("CARGO_PKG_NAME"), shell, &mut io::stdout());
+    let buffer = Vec::new();
+    let mut cursor = Cursor::new(buffer);
+    Self::app().gen_completions_to(env!("CARGO_PKG_NAME"), shell, &mut cursor);
+    let buffer = cursor.into_inner();
+    let text = String::from_utf8(buffer).expect("Clap completion not UTF-8");
+    println!("{}", text.trim());
 
     Ok(())
   }