Teach difftool to exit when a diff tool returns a non-zero exit
code when either --trust-exit-code is specified or
difftool.trustExitCode is true.
Forward exit codes from invoked diff tools to the caller when
--trust-exit-code is used.
Suggested-by: Adri Farr <14farresa@gmail.com> Helped-by: Johannes Sixt <j6t@kdbg.org> Signed-off-by: David Aguilar <davvid@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>