fix(branching): cross-repo PR builds fail

[tbcrawford]

Note

Medium Risk
Changes how the current git ref is determined in CI and alters commit-count resolution fallback behavior, which can affect version strings produced on build pipelines.

Overview
Fixes version calculation in CI for forked/cross-repository GitHub pull requests where GITHUB_HEAD_REF points to a branch that doesn’t exist in the checked-out repo. Branch.currentRef now falls back to a synthetic Ref named from GITHUB_HEAD_REF (otherwise HEAD) instead of failing, and BranchList.commitCountBetween falls back to resolving HEAD when the target ref can’t be resolved.

Adds a new functional test suite (CrossRepositoryPullRequestSpec) covering: missing local branch with GITHUB_HEAD_REF, preferring GITHUB_HEAD_REF over GITHUB_REF_NAME, and falling back to GITHUB_REF_NAME when GITHUB_HEAD_REF is empty.

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Cursor Bugbot has reviewed your changes and found 1 potential issue.

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SyntheticRef.getTarget() violates JGit Ref interface contract

Medium Severity

SyntheticRef.getTarget() returns null, but the JGit Ref interface contract specifies that when isSymbolic() is false, getTarget() must return this. All standard JGit non-symbolic ref implementations (e.g., ObjectIdRef) follow this contract. Returning null here could cause a NullPointerException if the ref is ever passed to JGit APIs or other code that relies on the documented non-null guarantee for non-symbolic refs.