pager: properly log pager exit code when signalled

pager: properly log pager exit code when signalled

When git invokes a pager that exits with non-zero the common case is
that we'll already return the correct SIGPIPE failure from git itself,
but the exit code logged in trace2 has always been incorrectly
reported[1]. Fix that and log the correct exit code in the logs.

Since this gives us something to test outside of our recently-added
tests needing a !MINGW prerequisite, let's refactor the test to run on
MINGW and actually check for SIGPIPE outside of MINGW.

The wait_or_whine() is only called with a true "in_signal" from from
finish_command_in_signal(), which in turn is only used in pager.c.

The "in_signal && !WIFEXITED(status)" case is not covered by
tests. Let's log the default -1 in that case for good measure.

1. The incorrect logging of the exit code in was seemingly copy/pasted
   into finish_command_in_signal() in ee4512ed481 (trace2: create new
   combined trace facility, 2019-02-22)

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>