Fix truncation of overly long post names.

Delete the old truncate_post_name; it was called too late (if url rewriting
is enabled which it is by default), didn't run (it tried to use + to concat
a Fixnum onto a String), and even with those problems fixed, didn't actually
shorten the string enough to use as a pathname.

Instead, apply simple string truncation at the point we generate the slug,
which is used in the filename and is the part that could be unboundedly
long. I arbitrarily chose 200 as the maximum length; even shorter might be
better (really long slugs are just visually ugly); it might also be nicer
to truncate at a hyphen boundary.

This fixes the rest of issue #773 (subissue 4).

lib/jekyll/migrators/tumblr.rb

@@ -34,7 +34,6 @@ module Jekyll
           post[:content] = html_to_markdown post[:content]
           post[:content] = add_syntax_highlights post[:content] if add_highlights
         end
-        post[:name] = truncate_post_name post[:name] if post[:name].size > 255
         File.open("_posts/tumblr/#{post[:name]}", "w") do |f|
           f.puts post[:header].to_yaml + "---\n" + post[:content]
         end
@@ -43,11 +42,6 @@ module Jekyll
 
     private
 
-    def self.truncate_post_name name
-      post = name.match(/^(.+)\.(.+)$/).captures
-      post[0][0..(-1 - post[1].size)] + post[1].size
-    end
-
     # Converts each type of Tumblr post to a hash with all required
     # data for Jekyll.
     def self.post_to_hash(post, format)
@@ -103,6 +97,7 @@ module Jekyll
       date = Date.parse(post['date']).to_s
       title = Nokogiri::HTML(title).text
       slug = title.downcase.strip.gsub(' ', '-').gsub(/[^\w-]/, '')
+      slug = slug.slice(0..200) if slug.length > 200
       {
         :name => "#{date}-#{slug}.#{format}",
         :header => {