difftool--helper: add explicit exit statement
difftool--helper: add explicit exit statement

git-difftool--helper returns a zero exit status unless
--trust-exit-code is in effect.  Add an explicit exit statement
to make this clearer.

Signed-off-by: David Aguilar <9de348c050f7cd1ca590883733c4e531ce610bf4@gmail.com>
Signed-off-by: Junio C Hamano <a6723cc3f76163bf7adb636a73ac3b0ceb3e6b9b@pobox.com>

destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git