Comparison Base Resolution

Use this skill when another workflow needs to compare a branch or working tree with the branch it is intended to merge into. This skill resolves the comparison refs and commit; the caller decides what paths and endpoint to diff.

Inputs

• $REVIEW_REF: the commit being reviewed. Default to HEAD.

Set $IS_LOCAL_REVIEW=true when the input is HEAD, or when a named local branch is the currently checked-out branch; otherwise set it to false. Callers use this explicit value to decide whether workspace changes belong to the review.

For HEAD or a named review branch, record $REVIEW_BRANCH when the ref identifies a local branch. Resolve its published destination using Git's push precedence: branch.<name>.pushRemote, then remote.pushDefault, then the branch's configured upstream remote. Store the remote name or URL usable by Git as $REVIEW_FETCH_SOURCE, normalize that remote's GitHub URL to owner/repository as $REVIEW_REPOSITORY, and store the remote branch name as $REVIEW_HEAD_BRANCH. Keep these values distinct: Git commands consume $REVIEW_FETCH_SOURCE, while GitHub PR metadata uses $REVIEW_REPOSITORY. The published branch may have a different name from the local branch. Without any configured destination, inspect configured remotes for an exact $REVIEW_BRANCH match and derive all three values; ask the user if multiple remotes match. A detached HEAD, commit input, or unpublished local branch may leave the published identity unset.

For a named review branch, prefer an exact local branch and set REVIEW_REF=refs/heads/$REVIEW_BRANCH. If there is no local branch and exactly one remote matches, set $REVIEW_FETCH_SOURCE to that remote, derive $REVIEW_REPOSITORY from its URL, set $REVIEW_HEAD_BRANCH=$REVIEW_BRANCH, fetch the branch into a dedicated ref, and use that ref as $REVIEW_REF:

git fetch --no-tags "$REVIEW_FETCH_SOURCE" "+refs/heads/$REVIEW_BRANCH:refs/comparison/head"
REVIEW_REF=refs/comparison/head