Don't remove + / - signs from diff emails

In the browser, we remove the + and - signs from the front of a diff
line because we add them in with CSS, so they aren't copied. We can't do
that in an email, because the CSS isn't supported, so we should keep
them in that case.

app/helpers/diff_helper.rb

@@ -51,9 +51,11 @@ module DiffHelper
     html.html_safe
   end
 
-  def diff_line_content(line)
+  def diff_line_content(line, email: false)
     if line.blank?
       " ".html_safe
+    elsif email
+      line.html_safe
     else
       line.sub(/^[\-+ ]/, '').html_safe
     end

app/views/projects/diffs/_line.html.haml

@@ -25,7 +25,7 @@
         %a{href: "##{line_code}", data: { linenumber: link_text }}
     %td.line_content.noteable_line{ class: type, data: (diff_view_line_data(line_code, diff_file.position(line), type) unless plain) }<
       - if email
-        %pre= diff_line_content(line.text)
+        %pre= diff_line_content(line.text, email: true)
       - else
         = diff_line_content(line.text)