diff 2.8.1: Some -I ignored?

[Akim Demaille]

I don't understand the following behavior of -I.  It seems to
differently with some characters, and I couldn't connect this behavior
with the documentation.  So I guess it's a bug, but it is so weird
that I suspect some king of magic feature I don't understand.

Notice that ignoring line matching "x" does the expected thing, while
it does not work with "~".  I tried with ":", it does the same.  I
have not tried the whole charset.

Thanks!

/tmp % eof foo bar                                               nostromo 10:12
: -------------------- foo ----------------------
cat > foo << \EOF
some text
some text
some text
some text
some text
some~text
some text
some text
some text
some text
some text
some text
EOF
: -------------------- bar ----------------------
cat > bar << \EOF
some text
some text
some text
some text
some text
somextext
some text
some text
some text
some text
some text
some text
EOF
: ------------------------------------------------------------
/tmp % diff -I x foo bar                                         nostromo 10:12
/tmp % diff -I '~' foo bar                                       nostromo 10:12
6c6
< some~text
---
> somextext
/tmp %                                                           nostromo Err 1