More bug fixes for completions (#2476)

* Use the cursor position for the span when between locations.

This fixes a bug where completing

    ls <TAB>

would result in replacing the entire line.

* Revert to the default update implementation.

Replacing the length of the elected value was intended to do replacement when
one moves inside a quote. The problem is that a long elected suggestion could
replace bits of a pipeline that are after the cursor. For example:

    ls <TAB> | get name | str collect

crates/nu-cli/src/completion/engine.rs

@@ -232,10 +232,10 @@ pub fn completion_location(line: &str, block: &Block, pos: usize) -> Vec<Complet
             // is after some character that would imply we're in the command position.
             let start = prev.span.end();
             if line[start..pos].contains(BEFORE_COMMAND_CHARS) {
-                vec![LocationType::Command.spanned(Span::unknown())]
+                vec![LocationType::Command.spanned(Span::new(pos, pos))]
             } else {
                 // TODO this should be able to be mapped to a command
-                vec![LocationType::Argument(command, None).spanned(Span::unknown())]
+                vec![LocationType::Argument(command, None).spanned(Span::new(pos, pos))]
             }
         } else {
             // Cursor is before any possible completion location, so must be a command

crates/nu-cli/src/shell/helper.rs

@@ -54,7 +54,7 @@ impl rustyline::completion::Completer for Helper {
     }
 
     fn update(&self, line: &mut rustyline::line_buffer::LineBuffer, start: usize, elected: &str) {
-        let end = (start + elected.len()).min(line.len());
+        let end = line.pos();
         line.replace(start..end, elected)
     }
 }