run-command: don't warn on SIGPIPE deaths

run-command: don't warn on SIGPIPE deaths

When git executes a sub-command, we print a warning if the
command dies due to a signal, but make an exception for
"uninteresting" cases like SIGINT and SIGQUIT (since the
user presumably just hit ^C).

We should make a similar exception for SIGPIPE, because it's
an expected and uninteresting return in most cases; it
generally means the user quit the pager before git had
finished generating all output.  This used to be very hard
to trigger in practice, because:

  1. We only complain if we see a real SIGPIPE death, not
     the shell-induced 141 exit code. This means that
     anything we run via the shell does not trigger the
     warning, which includes most non-trivial aliases.

  2. The common case for SIGPIPE is the user quitting the
     pager before git has finished generating all output.
     But if the user triggers a pager with "-p", we redirect
     the git wrapper's stderr to that pager, too.  Since the
     pager is dead, it means that the message goes nowhere.

  3. You can see it if you run your own pager, like
     "git foo | head". But that only happens if "foo" is a
     non-builtin (so it doesn't work with "log", for
     example).

However, it may become more common after 86d26f2, which
teaches alias to re-exec builtins rather than running them
in the same process. This case doesn't trigger (1), as we
don't need a shell to run a git command. It doesn't trigger
(2), because the pager is not started by the original git,
but by the inner re-exec of git. And it doesn't trigger (3),
because builtins are treated more like non-builtins in this
case.

Given how flaky this message already is (e.g., you cannot
even know whether you will see it, as git optimizes out some
shell invocations behind the scenes based on the contents of
the command!), and that it is unlikely to ever provide
useful information, let's suppress it for all cases of
SIGPIPE.

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