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 <davvid@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-difftool--helper.sh

@@ -94,3 +94,5 @@ else
 		shift 7
 	done
 fi
+
+exit 0