D3859: unlinkpath: make empty directory removal optional (issue5901) (issue5826)

[Yuya Nishihara]

Queued, thanks.

> +  $ NO_RM=--config=experimental.removeemptydirs=0
> +  $ isdir() { if [[ -d $1 ]]; then echo yes; else echo no; fi }
> +  $ isfile() { if [[ -f $1 ]]; then echo yes; else echo no; fi }

changed to `[ ... ]` since `[[ ... ]]` requires bash.