Pre-truncate excerpt before running replacement regexes

Also fixes <ol> normalization which was previously a noop.

app/helpers/excerpt_helper.rb

@@ -3,9 +3,10 @@ module ExcerptHelper
     return "" if content.blank?
 
     text = content.respond_to?(:to_plain_text) ? content.to_plain_text : content.to_s
+    text = text.first(length * 2)
     text = text.gsub(/^>\s*(.*)$/m, '> \1')
     text = text.gsub(/^\s*[-+]\s*(.*)$/m, '• \1')
-    text = text.gsub(/^\d+\.\s*(.*)$/m) { |m| m }
+    text = text.gsub(/^\s*(\d+\.)\s*(.*)$/m, '\1 \2')
     text = text.gsub(/\s+/, " ").strip
     text.truncate(length)
   end

test/helpers/excerpt_helper_test.rb

@@ -11,7 +11,8 @@ class ExcerptHelperTest < ActionView::TestCase
   end
 
   test "ol" do
-    assert_excerpt("99. Hello world", "99. Hello world")
+    assert_excerpt("99. Hello world", "99.    Hello world")
+    assert_excerpt("99. Hello world", "  99.    Hello world")
   end
 
   test "large spaces" do