Merge branch 'tb/commit-graph-tests'

Test updates.

* tb/commit-graph-tests:
  t/lib-commit-graph.sh: avoid sub-shell in `graph_git_behavior()`
  t5328: avoid top-level directory changes
  t5318: avoid top-level directory changes
  t/lib-commit-graph.sh: avoid directory change in `graph_git_behavior()`
  t/lib-commit-graph.sh: allow `graph_read_expect()` in sub-directories