svn diff: file marked as binary type
You can get diff even for a file marked as binary by using --force.
svn diff --force path/to/file
You can use the Subversion property svn:mime-type to set an explicit mimetype on the file:
svn propset svn:mime-type 'text/plain' path/to/file
Alternatively, you can delete this property (since Subversion assumes plaintext, otherwise) using:
svn propdel svn:mime-type path/to/file