Add support for showing difference when line endings are different.

[BernH4]

Currently when only the line endings differ (Unix \n and Windows \r\n for example) DirDiff says that the files differ but the lines are all collapsed (like when there is really no difference)

With normal vimdiff i can use following command when already inside diff:
:windo e! ++ff=unix

or before starting vimdiff:
$ vimdiff --cmd 'set fileformats=unix' file1 file2

dos2unix and unix2dos commands can be used to reproduce this issue.