"Ctrl + P and O more or less do the same thing as Ctrl + Right and left

Also remove a warning about a useless mut"

src/shell/history.rs

@@ -94,7 +94,7 @@ pub fn get_hist_from_number(number: &i32) -> Option<String> {
 }
 
 pub fn treat_history_callback(line: &str) -> Option<String> {
-    let mut mystring = String::from(line);
+    let mystring = String::from(line);
     let temp = mystring.split_whitespace().collect::<Vec<&str>>();
     let mystring = temp.first().unwrap_or(&line);
     let mut chars = mystring.chars();