send-pack: complain about "expecting report" with --helper-status

When pushing to a server which erroneously omits the final ref-status
report, the client side should complain about the refs for which we
didn't receive the status (because we can't just assume they were
updated). This works over most transports like ssh, but for http we'll
print a very misleading "Everything up-to-date".

It works for ssh because send-pack internally sets the status of each
ref to REF_STATUS_EXPECTING_REPORT, and then if the server doesn't tell
us about a particular ref, it will stay at that value. When we print the
final status table, we'll see that we're still on EXPECTING_REPORT and
complain then.

But for http, we go through remote-curl, which invokes send-pack with
"--stateless-rpc --helper-status". The latter option causes send-pack to
return a machine-readable list of ref statuses to the remote helper. But
ever since its inception in de1a2fdd38 (Smart push over HTTP: client
side, 2009-10-30), the send-pack code has simply omitted mention of any
ref which ended up in EXPECTING_REPORT.

In the remote helper, we then take the absence of any status report
from send-pack to mean that the ref was not even something we tried to
send, and thus it prints "Everything up-to-date". Fortunately it does
detect the eventual non-zero exit from send-pack, and propagates that in
its own non-zero exit code. So at least a careful script invoking "git
push" would notice the failure.  But sending the misleading message on
stderr is certainly confusing for humans (not to mention the
machine-readable "push --porcelain" output, though again, any careful
script should be checking the exit code from push, too).

Nobody seems to have noticed because the server in this instance has to
be misbehaving: it has promised to support the ref-status capability
(otherwise the client will not set EXPECTING_REPORT at all), but didn't
send us any. If the connection were simply cut, then send-pack would
complain about getting EOF while trying to read the status. But if the
server actually sends a flush packet (i.e., saying "now you have all of
the ref statuses" without actually sending any), then the client ends up
in this confused situation.

The fix is simple: we should return an error message from "send-pack
--helper-status", just like we would for any other error per-ref error
condition (in the test I included, the server simply omits all ref
status responses, but a more insidious version of this would skip only
some of them).

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

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