The following commands work on OS X, and cause darcs to read bigtempfile before
telling you that it is to be added.

For other unices, use --bytes= instead of -c, the --darcs-2 to init doesn't
matter, but I wanted to demonstrate that darcs-2 repos do not fix this issue.

time head -c 17179869184 /dev/zero > bigtempfile
darcs init --darcs-2
darcs add bigtempfile
darcs whats -s

You'll notice this takes much longer than it should.  I'm pretty sure this is
because adding the file puts it pending and the summary needs to know if this is
a binary or non-binary patch.  Determining that a file is not binary requires
reading the whole file in smart_diff and diff_files because has_bin calls
is_funky to look at every char in the file checking for characters that are
typical in binary files.