bash: prompt: pin wdbase to one level above instead

Fixes inconsistency between current directory being the repository, and
one level deep in the repository, both of which used to have the last
element highlighted.
bash: prompt: experimentally fix the same issue by using empty wdtail

An alternative fix to that in 5ea086a.

Side note: I just realized that the whole wdbase hack screws with
collapse if one is really deep inside a repository. This could probably
be fixed in a way similar to:

	if (( ${#wdtail} > maxwidth )); then
		wdhead=${wdhead##*/}
		if [[ $wdhead == */* ]]; then
			... duplicate the 'while' loop ...
		fi
		collapsed=1
	elif ...
bash: prompt: wdhead will always end up being '/', hardcode it
Revert "bash: prompt: experimentally fix the same issue by using empty wdtail"

This reverts commit 4bd69f12ab7c403a53d78e24e1f43babe99d1b1a.

I decided I liked the other solution better. But the other comment still
applies, will need to fix later.