process_full_diff: Don't reencode diffs which are already UTF-8 (#369)

Author: tbrand

lib/git/diff.rb

@@ -127,10 +127,11 @@ def process_full_diff
         }
         final = {}
         current_file = nil
-        full_diff_utf8_encoded = @full_diff.encode("UTF-8", "binary", {
-			:invalid => :replace,
-			:undef => :replace
-		})
+        if @full_diff.encoding.name != "UTF-8"
+          full_diff_utf8_encoded = @full_diff.encode("UTF-8", "binary", { :invalid => :replace, :undef => :replace })
+        else
+          full_diff_utf8_encoded = @full_diff
+        end
         full_diff_utf8_encoded.split("\n").each do |line|
           if m = /^diff --git a\/(.*?) b\/(.*?)/.match(line)
             current_file = m[1]