run_processes_parallel(): fix order of sigpipe handling

In commit ec0becacc9 (run-command: add stdin callback for
parallelization, 2026-01-28), we taught run_processes_parallel() to
ignore SIGPIPE, since we wouldn't want a write() to a broken pipe of one
of the children to take down the whole process.

But there's a subtle ordering issue. After we ignore SIGPIPE, we call
pp_init(), which installs its own cleanup handler for multiple signals
using sigchain_push_common(), which includes SIGPIPE. So if we receive
SIGPIPE while writing to a child, we'll trigger that handler first, pop
it off the stack, and then re-raise (which is then ignored because of
the SIG_IGN we pushed first).

But what does that handler do? It tries to clean up all of the child
processes, under the assumption that when we re-raise the signal we'll
be exiting the process!

So a hook that exits without reading all of its input will cause us to
get SIGPIPE, which will put us in a signal handler that then tries to
kill() that same child.

This seems to be mostly harmless on Linux. The process has already
exited by this point, and though kill() does not complain (since the
process has not been reaped with a wait() call), it does not affect the
exit status of the process.

However, this seems not to be true on all platforms. This case is
triggered by t5401.13, "pre-receive hook that forgets to read its
input". This test fails on NonStop since that hook was converted to the
run_processes_parallel() API.

We can fix it by reordering the code a bit. We should run pp_init()
first, and then push our SIG_IGN onto the stack afterwards, so that it
is truly ignored while feeding the sub-processes.

Note that we also reorder the popping at the end of the function, too.
This is not technically necessary, as we are doing two pops either way,
but now the pops will correctly match their pushes.

This also fixes a related case that we can't test yet. If we did have
more than one process to run, then one child causing SIGPIPE would cause
us to kill() all of the children (which might still actually be
running). But the hook API is the only user of the new feed_pipe
feature, and it does not yet support parallel hook execution. So for now
we'll always execute the processes sequentially. Once parallel hook
execution exists, we'll be able to add a test which covers this.

Reported-by: Randall S. Becker <rsbecker@nexbridge.com>
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>