AXDiff is a graphical file comparison and merge utility.  It allows
you to see files side by side with differences between the files
highlighted.  You may move within the files looking at the differences
within context of the rest of the file.  A merged file can be created
which contains the lines which do not differ between files along with
user-selected lines from the sections which differ.  You can select
either no part, either part, or both parts of the sections which
differ to be written to the merged file.  Selectable fonts, sizes, and
other options too.