Fix rouge-ruby#1022 by supporting diffs other than unified. (#36)

In addition to handling diffs done without the `-u` flag, this also
more closely matches `git diff` output for unified diffs by treating `@@
[reference ] @@` as punctuation and the rest of the line as text, making
`---` punctuation, and making the diff/index lines at the top headings.

lib/rouge/lexers/diff.rb

@@ -19,14 +19,18 @@ def self.detect?(text)
 
       state :root do
         rule(/^ .*$\n?/, Text)
-        rule(/^---$\n?/, Text)
+        rule(/^---$\n?/, Punctuation)
+        rule(/^[+>]+.*$\n?/, Generic::Inserted)
         rule(/^\+.*$\n?/, Generic::Inserted)
-        rule(/^-+.*$\n?/, Generic::Deleted)
+        rule(/^[-<]+.*$\n?/, Generic::Deleted)
         rule(/^!.*$\n?/, Generic::Strong)
-        rule(/^@.*$\n?/, Generic::Subheading)
         rule(/^([Ii]ndex|diff).*$\n?/, Generic::Heading)
+        rule(/^(@@[^@]*@@)([^\n]*\n)/) do
+          groups Punctuation, Text
+        end
+        rule(/^\w.*$\n?/, Punctuation)
         rule(/^=.*$\n?/, Generic::Heading)
-        rule(/.*$\n?/, Text)
+        rule(/\s.*$\n?/, Text)
       end
     end
   end