tree: handle format of START/BEGIN inside of functions

[rafiss]

If a transaction is started in a BEGIN ATOMIC block of a function or procedure, it must be formatted with START rather than BEGIN. Otherwise the function body cannot be parsed.

fixes #109561
Release note: None

[cockroach-teamcity]

This change is 

[mgartner]

LGTM. Thanks for fixing this one!

[rafiss]

tftr!

bors r+

[craig]

Build succeeded:

• Bazel Essential 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 7ae12d1 to blathers/backport-release-23.1-109618: 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 23.1.x failed. See errors above.

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