core/diff: Fix attribute mismatch with tags.%

[catsby]

This is the start of a patch to fix the acceptance test TestAccAWSInstance_forceNewAndTagsDrift. Right now it's failing like so:

--- FAIL: TestAccAWSInstance_forceNewAndTagsDrift (103.36s)
    testing.go:247: Step 1 error: Error applying: 1 error(s) occurred:

        * aws_instance.foo: diffs didn't match during apply. This is a bug with Terraform and should be reported as a GitHub Issue.

        Please include the following information in your report:

            Terraform Version: 0.7.0
            Resource ID: aws_instance.foo
            Mismatch reason: attribute mismatch: tags.%

(from https://travis-ci.org/hashicorp/terraform/jobs/136731270)

It's a regression test that was introduced in #4749 . With the recent addition of native lists, the state syntax for maps changed(?) from thing.# to thing.%, and so the safety provided in #4749 was lost (sort of).

This is my first foray into core-land so it's not clear to me if the test case is sufficient, and if the additional HasSuffix is enough of we need more of those in other place. I poke with @phinze and we started this at the end of the day, so I wanted to get the PR open to preserve some context.

[jen20]

Thanks @catsby! This look correct to me. Great catch on your first foray into core land! 👍

[ghost]

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