Cover git update branch selection

[codeforester]

Summary

• Add direct BATS coverage for _git_expected_update_branch choosing origin/main, origin/master, and the graceful master fallback.
• Add direct BATS coverage for _git_update_repo_finish log cleanup and RETURN trap preservation.
• Teach _git_expected_update_branch to check remote origin/main and origin/master refs before falling back to local branches.

Validation

• bats lib/bash/git/tests/lib_git.bats
• ./bin/base-test
• git diff --check

Fixes #301