credential: ignore SIGPIPE when writing to credential helpers

credential: ignore SIGPIPE when writing to credential helpers

The credential subsystem can trigger SIGPIPE when writing to an
external helper if that helper closes its stdin before reading the
whole input. Normally this is rare, since helpers would need to read
that input to make a decision about how to respond, but:

1. It's reasonable to configure a helper which only handles "get"
   while ignoring "store".  Such a handler might not read stdin
   for "store", thereby rapidly closing stdin upon helper exit.

2. A broken or misbehaving helper might exit immediately. That's an
   error, but it's not reasonable for it to take down the parent Git
   process with SIGPIPE.

Even with such a helper, seeing this problem should be rare. Getting
SIGPIPE requires the helper racily exiting before we've written the
fairly small credential output.

Signed-off-by: Erik E Brady <brady@cisco.com>
Reviewed-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>