Outdent at one line in a multi-line blockquote should outdent only the one line

[stevengharris]

If you have a blockquote with multiple lines like:

This is a multiline
blockquote

and you try to outdent one line, both get outdented. This happens because Enter in a blockquote just creates a new paragraph within the blockquote, so outdent is just removing the one blockquote that surrounds both <p> elements. This is probably better done by having enter produce a separate blockquote for each <p>, especially now that multidenting allows you to select multiple dented paragraphs and take an action on them collectively.