lint: have git-subtree-check warn about backportability #35686

A subtree update PR is easier to backport when its merge commit is stacked on the previous subtree merge instead of on master.

Have git-subtree-check.sh warn when the latest subtree merge does not follow that structure. This is not always practical, so it's a warning, not a failure.

Example without warning:

test/lint/git-subtree-check.sh src/ipc/libmultiprocess 66b4e30e

Example with warning:
test/lint/git-subtree-check.sh src/ipc/libmultiprocess 02afa661
```sh

The check only runs when the commit (`COMMIT`, or `HEAD`) is the subtree merge itself. Since CI performs a merge with master, it skips this check.

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For details see: https://corecheck.dev/bitcoin/bitcoin/pulls/35686.

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