merge-ll: report an error when reading external merge results fails

If we can't read an external merge driver's output, ll_ext_merge()
leaves the result buffer as NULL but returns a status based only on the
driver's exit code. So a driver which exits successfully can cause us to
return LL_MERGE_OK without a result.

Most callers of ll_merge() check for a NULL buffer in addition to an
error return, so they're fine. But rerere's merge() checks only the
return value, and may write out the (incorrect) empty result as the
recorded resolution.

Let's return LL_MERGE_ERROR when read_mmfile() fails, regardless of the
driver's exit status, to make it clear that the returned value is not
valid.

Our test is a little funny; the bad case happens when reading back the
file happens to fail. That can happen due to system errors, but of
course we want it to be deterministic. We can make that happen by
removing the result file. But if we configure a driver that always does
that, we'd never record a rerere result in the first place! So we
instead create a driver that "breaks" the read only when we instruct it
to do so.

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