Drupal Redirect User Login Page
You might want to have a look a the Secure Pages module, which is:


  A small process which will redirect the required pages to a SSL version of the page.


You can easily set up the user/login path as an SSL-required page, and the redirect will be handled for you.

If you want to do it manually, a rule like this in .htaccess would probably work:

RewriteCond %{SERVER_PORT} !^443$
RewriteRule ^user$ https://%{SERVER_NAME}/user [R=301,L]


That would need to go above Drupal's main rewrite rule for index.php.

It's worth bearing in mind you'll need to do this for the user/login path as well as that can be accessed directly.