Fix av stack adopt to detect the trunk branch first

[draftcode]

Without this, if the trunk branch points to a merge commit, av stack
adopt won't work.

[aviator-app]

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[aviator-app]

FlexReview Summary

Based on the code complexity and the author's expertise score, these are the suggested reviewers:

• @jainankit (current review load: 3)

See the list of alternate reviewers in the detailed breakdown below.

Detailed Breakdown

Author’s expertise score for the modified files:

• internal/treedetector/detector.go (1.00)

Files	Reviewers
internal/treedetector/detector.go †	@jainankit (score: 1.00, current review load: 3)

† Indicates that the file doesn't need an expert review. (?)

See full breakdown of the reviewers on the Aviator webapp.