Fix two data races in the branch names cache

What does this MR do?

Expiry of the branch name cache could race with checking inclusion, in such a way that a branch may be marked as non-existent when it does in fact exist.

This MR uses Redis transactions to atomically take the existence of the set at the same time as the smembers / sismembers call, so we can distinguish between "an empty value exists in the cache" and "the cache is empty".

Does this MR meet the acceptance criteria?

Conformity

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  • I have included a changelog entry.
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Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
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Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
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Related to #326066 (closed)