[Resolve #632] Prevent before_update on no-ops

[devenney]

• Create change sets before any update operation to test for changes
• Only execute before_update hook when change set is empty
• Update tests to mock non-empty change sets

Resolves #632.

Co-authored-by: Amy Robertson [email protected]
Co-authored-by: Chip Wolf [email protected]
Co-authored-by: David Munn [email protected]

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Signed-off-by: Brendan Devenney [email protected]

Resolves the issue of executing before_update before checking if there are updates. When there are no updates to perform, Sceptre exits without executing after_update. The findings in #632 suggest before_update's behaviour is unintended rather than after_update.

If you feel it's worth adding unit tests to test that the hooks are/aren't run given the relevant Stack states, let me know. Presumably that would constitute a larger piece of work as it touches all hooks (create, update, delete).

PR Checklist

• Wrote a good commit message & description [see guide below].
• Commit message starts with [Resolve #issue-number].
• Added/Updated unit tests.
• Added/Updated integration tests (if applicable).
• All unit tests (make test) are passing.
• Used the same coding conventions as the rest of the project.
• The new code passes flake8 (make lint) checks.
• The PR relates to only one subject with a clear title.
  and description in grammatically correct, complete sentences.

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• Before merge squash related commits.

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