difftool: add support for --trust-exit-code

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>