sha1.c: replace white/blacklist with allow/denylist

[derrickstolee]

The words "whitelist" and "blacklist" have cultural implications that are not inclusive. The words "allowlist" and "denylist" are self-descriptive without requiring cultural implications to understand the words in context.

A similar set of changes were sent to the Git project, but in those cases the words could be removed by rephrasing the sentences that used them, increasing clarity. In this example, the terms are used concretely enough that rephrasing would decrease clarity. Thus, a relatively mechanical replacement was applied.

These changes only affect comment lines, nothing functional.

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I was redirected here after trying to make similar changes in the Git codebase [1], forgetting that this file is maintained in this repository.

Thanks for considering this change!

[1] https://lore.kernel.org/git/[email protected]