[m-rev.] diff: fix bugs with existential types & secondary tags

[Ralph Becket]

> If you keep the file tab-free, someone else (e.g. me) can edit the
file by
> setting ts=4, working away normally (which will insert 4-space tabs),
and
> then restore the file to its original tab-free state with "detab -4".

Vim users can `set expandtab' to cause tabs to be inserted using spaces;

`:retab' will replace all tab characters in a file with the requisite
number
of spaces.

-- Ralph
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