receive-pack: send pack-processing stderr over sideband

receive-pack: send pack-processing stderr over sideband

Receive-pack invokes either unpack-objects or index-pack to
handle the incoming pack. However, we do not redirect the
stderr of the sub-processes at all, so it is never seen by
the client. From the initial thread adding sideband support,
which is here:

  http://thread.gmane.org/gmane.comp.version-control.git/139471

it is clear that some messages are specifically kept off the
sideband (with the assumption that they are of interest only
to an administrator, not the client). The stderr of the
subprocesses is mentioned in the thread, but it's unclear if
they are included in that group, or were simply forgotten.

However, there are a few good reasons to show them to the
client:

  1. In many cases, they are directly about the incoming
     packfile (e.g., fsck warnings with --strict, corruption
     in the packfile, etc). Without these messages, the
     client just gets "unpacker error" with no extra useful
     diagnosis.

  2. No matter what the cause, we are probably better off
     showing the errors to the client. If the client and the
     server admin are not the same entity, it is probably
     much easier for the client to cut-and-paste the errors
     they see than for the admin to try to dig them out of a
     log and correlate them with a particular session.

  3. Users of the ssh transport typically already see these
     stderr messages, as the remote's stderr is copied
     literally by ssh. This brings other transports (http,
     and push-over-git if you are crazy enough to enable it)
     more in line with ssh. As a bonus for ssh users,
     because the messages are now fed through the sideband
     and printed by the local git, they will have "remote:"
     prepended and be properly interleaved with any local
     output to stderr.

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