Merge branch 'sg/bash-prompt-dirty-orphan'

Produce correct "dirty" marker for shell prompts, even when we
are on an orphan or an unborn branch.

* sg/bash-prompt-dirty-orphan:
  bash prompt: indicate dirty index even on orphan branches
  bash prompt: remove a redundant 'git diff' option
  bash prompt: test dirty index and worktree while on an orphan branch