Use instance variable for link

The merge request URL is pre-available as target_url
and is also used in the footer to link to the same destination

This change reuses that, as suggested by @vij

app/views/notify/new_merge_request_email.html.haml

 %p.details
   = html_escape(_('%{userLinkStart}%{user}%{linkEnd} created a %{mrLinkStart}merge request%{linkEnd}:')) % {userLinkStart: "<a href=\"#{user_url(@merge_request.author)}\">".html_safe,
                                                                                                             user: @merge_request.author_name,
-                                                                                                            mrLinkStart: "<a href=\"#{merge_request_url(@merge_request)}\">".html_safe,
+                                                                                                            mrLinkStart: "<a href=\"#{@target_url}\">".html_safe,
                                                                                                             linkEnd: '</a>'.html_safe}
 
 %p