merge-ort: drop custom err() function

merge-ort: drop custom err() function

The merge-ort code has an err() function, but it's really just error()
in disguise. It differs in two ways:

  1. It takes a "struct merge_options" argument. But the function
     completely ignores it! We can simply remove it.

  2. It formats the error string into a strbuf, prepending "error: ",
     and then feeds the result into error(). But this is wrong! The
     error() function already adds the prefix, so we end up with:

        error: error: Failed to execute internal merge

So let's just drop this function entirely and call error() directly, as
the functions are otherwise identical (note that they both always return
-1).

Presumably nobody noticed the bogus messages because they are quite hard
to trigger (they are mostly internal errors reading and writing
objects). However, one easy trigger is a custom merge driver which dies
by signal; we have a test already here, but we were not checking the
contents of stderr.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>