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This pull-request has been approved by: costinm

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I just had a hassle trying to get vendor to update. Apparently if I do a sequence like "rm -rf vendor; git checkout vendor; git fetch upstream; git rebase upstream/master; make pull" then vendor doesn't update the next time I try running the last 3 steps. If I run "git status" within vendor/ then it reports I have a detached head. I'm not sure if that's expected.

I've lost track of what checks might have been added or to-do for either presubmit or postsubmit. Anyway, something like this comes to mind as a way to only do a diff when the file have changed since the last check though I haven't checked if git lets the OS handle the timestamps of source files or instead sets them to reflect when the file was last changed in the repo.

- init: submodule check-go-version $(ISTIO_OUT)/istio_is_init
+ init: submodule $(ISTIO_OUT)/vendor_checked check-go-version $(ISTIO_OUT)/istio_is_init

$(ISTIO_OUT)/vendor_checked: Gopkg.toml Gopkg.lock | $(ISTIO_OUT)
	diff -q Gopkg.toml vendor/ || echo "vendor/ out-of-sync, run 'make pull' and/or seek on-call help" && exit 1
	diff -q Gopkg.lock vendor/ || echo "vendor/ out-of-sync, run 'make pull' and/or seek on-call help" && exit 1
	touch $(ISTIO_OUT)/vendor_checked