fetch-pack: ignore SIGPIPE when writing to index-pack

fetch-pack: ignore SIGPIPE when writing to index-pack

When fetching, we send the incoming pack to index-pack (or
unpack-objects) via the sideband demuxer. If index-pack hits an error
(e.g., because an object fails fsck), then it will die immediately. This
may cause us to get SIGPIPE on the fetch, as we're still trying to write
pack contents from the sideband demuxer (which is typically a thread,
and thus takes down the whole fetch process).

You can see this in action with:

  ./t5702-protocol-v2.sh --stress --run=59

which ends with (wrapped for readability):

  test_must_fail: died by signal 13: git -c protocol.version=2 \
    -c transfer.fsckobjects=1 -c fetch.uriprotocols=http,https \
    clone http://127.0.0.1:5708/smart/http_parent http_child
  not ok 59 - packfile-uri with transfer.fsckobjects fails on bad object

This is mostly cosmetic. The actual error of interest (in this case, the
object that failed the fsck check) comes from index-pack straight to
stderr, so the user still sees it. They _might_ even see fetch-pack
complaining about index-pack failing, because the main thread is racing
with the sideband-demuxer. But they'll definitely see the signal death
in the exit code, which is what the test is complaining about.

We can make this more predictable by just ignoring SIGPIPE. The sideband
demuxer uses write_or_die(), so it will notice and stop (gracefully,
because we hook die_routine() to exit just the thread). And during this
section we're not writing anywhere else where we'd be concerned about
SIGPIPE preventing us from wasting effort writing to nowhere.

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