update.sh: don't print "checkout branch" message.

[MikeMcQuaid]

• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't other open Pull Requests for the same change?
• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes? Here's an example.
• Have you successfully run brew tests with your changes locally?

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It’ll only get printed for people getting updated to tags now and these
are people who haven’t run a dev-cmd so we want to air on the side of
telling them less stuff that will confuse them and assume people who
have manually made another git branch will know how to get back to it.

See #1062, #1072,