ttljob: fix a range edge case

[otan]

See individual commits for details.

Refs: #81208

[cockroach-teamcity]

This change is 

[rafiss]

lgtm.. but there probably are other folks out there who know range encoding better than me, so idk if there's a different way

[otan]

i looked at how SHOW RANGES FOR TABLE worked and uh yeah this is what i found :D!

bors r=rafiss

[craig]

Build succeeded:

• GitHub CI (Cockroach)

[blathers-crl]

Encountered an error creating backports. Some common things that can go wrong:

1. The backport branch might have already existed.
2. There was a merge conflict.
3. The backport branch contained merge commits.

You might need to create your backport manually using the backport tool.

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error creating merge commit from 036f3f5 to blathers/backport-release-22.1.0-81207: POST https://api.github.com/repos/cockroachdb/cockroach/merges: 409 Merge conflict []

you may need to manually resolve merge conflicts with the backport tool.

Backport to branch 22.1.0 failed. See errors above.

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