apidiff only fails on incompatible changes (#127)

internal/kokoro/check_incompat_changes.sh

@@ -34,8 +34,8 @@ for dir in $V1_DIRS $V1_SUBDIRS; do
 
   # TODO(deklerk) there's probably a nicer way to do this that doesn't require
   # two invocations
-  if ! apidiff /tmp/pkg.master $pkg | (! read); then
-    apidiff /tmp/pkg.master $pkg
+  if ! apidiff -incompatible /tmp/pkg.master $pkg | (! read); then
+    apidiff -incompatible /tmp/pkg.master $pkg
     exit 1
   fi
 done