test: add integration tests for rebase --skip and robustness features

- rebase_skip_conflicted_branch: skip with conflict, verify branch
  restoration and remaining branch processing
- rebase_skip_no_state: --skip with no state file returns error
- rebase_skip_then_verify_chain_status: verify chain status after skip
- rebase_continue_after_external_abort: detect git rebase --abort
  run directly by user, verify error message
- rebase_continue_with_deleted_branch: handle branch deleted during
  chain rebase, verify warning and skip

Author: dashed