Index: lib/redmine/unified_diff.rb
===================================================================
--- lib/redmine/unified_diff.rb	(revision 11169)
+++ lib/redmine/unified_diff.rb	(working copy)
@@ -35,7 +35,7 @@
           # TODO: UTF-16 and Japanese CP932 which is imcompatible with ASCII
           #       In Japan, diffrence between file path encoding
           #       and file contents encoding is popular.
-          line.force_encoding('ASCII-8BIT')
+          line.force_encoding('UTF-8')
         end
         unless diff_table.add_line line
           line.force_encoding(line_encoding) if line_encoding