- Switch example plugin URLs from git+ssh:// to git+https:// so tests
  pass in CI without SSH keys
- Use Ref (branch/tag) for --branch flag instead of Rev (commit hash),
  since --depth 1 --branch doesn't work with arbitrary commit SHAs
- Include Ref field in Hash() to avoid false duplicate detection when
  plugins differ only by ref

The testscript framework strips PATH to only the testscript bin dir,
so the git binary isn't available when TestExamples runs devbox as a
subprocess. The git plugin examples are already covered by TestDevbox
shell plan tests which run in-process with the normal PATH.