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

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 <redacted>
Helped-by: Johannes Sixt <redacted>
Signed-off-by: David Aguilar <redacted>
Signed-off-by: Junio C Hamano <redacted>