copy: remove additional spaces when quoting already-quoted lines

When quoting a mail that already contains quoted parts the default indent_string will produce additional whitespace.

Example:

# Original text
foo
> bar

# Quoted by mutt
> foo
> > bar

I would like to remove the space between the two quote signs:

# Quoted by mutt
> foo
>> bar

If this is something that makes sense for the maintainers I can write the patch for it.