update.sh: tweak tag handling.

[MikeMcQuaid]

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• Don't let the UPSTREAM_TAG variable bleed into future repository checks.
• Even if the tag branch is an ancestor of the tag ensure that it's forced back to the tag anyway.

This will go into 1.0.1 but I'll hold off for a bit until we get any user reports (should only affect Homebrew developers and even then it's kinda tricky to reproduce).

Fixes #1062.

[MikeMcQuaid]

First report in #998 (comment). Figuring out whether they had "developer mode" enabled.