fetch: ignore SIGPIPE during network operation

The default SIGPIPE behavior can be useful for a command that generates
a lot of output: if the receiver of our output goes away, we'll be
notified asynchronously to stop generating it (typically by killing the
program).

But for a command like fetch, which is primarily concerned with
receiving data and writing it to disk, an unexpected SIGPIPE can be
awkward. We're already checking the return value of all of our write()
calls, and dying due to the signal takes away our chance to gracefully
handle the error.

On Linux, we wouldn't generally see SIGPIPE at all during fetch. If the
other side of the network connection hangs up, we'll see ECONNRESET. But
on OS X, we get a SIGPIPE, and the process is killed. This causes t5570
to racily fail, as we sometimes die by signal (instead of the expected
die() call) when the server side hangs up.

Let's ignore SIGPIPE during the network portion of the fetch, which will
cause our write() to return EPIPE, giving us consistent behavior across
platforms.

This fixes the test flakiness, but note that it stops short of fixing
the larger problem. The server side hit a fatal error, sent us an "ERR"
packet, and then hung up. We notice the failure because we're trying to
write to a closed socket. But by dying immediately, we never actually
read the ERR packet and report its content to the user. This is a (racy)
problem on all platforms. So this patch lays the groundwork from which
that problem might be fixed consistently, but it doesn't actually fix
it.

Note the placement of the SIGPIPE handling. The absolute minimal change
would be to ignore SIGPIPE only when we're writing. But twiddling the
signal handler for each write call is inefficient and maintenance
burden. On the opposite end of the spectrum, we could simply declare
that fetch does not need SIGPIPE handling, since it doesn't generate a
lot of output, and we could just ignore it at the start of cmd_fetch().

This patch takes a middle ground. It ignores SIGPIPE during the network
operation (which is admittedly most of the program, since the actual
network operations are all done under the hood by the transport code).
So it's still pretty coarse.

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

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