Fix gh helper output variable shadowing

[codeforester]

Summary

• fix gh_detect_default_branch and gh_repo_default_branch pass-by-name shadowing
• add regression coverage for callers that use default_branch as the result variable name

Fixes #122

Validation

• RED: bats lib/bash/gh/tests/lib_gh.bats failed on the new default_branch result-variable regression tests
• bats lib/bash/gh/tests/lib_gh.bats
• tests/lint-warnings.sh
• git diff --check
• tests/validate.sh