bash: prompt: fix collapse when wdtail has several items

Test case: /home/grawity/pkg/abs/telepathy-mission-control/git/src/telepathy-mission-control/tests/twisted/tools/servicedir-uninstalled
bash: prompt: fix hardcoded wdhead

I was confused there. $wdhead will end up empty in this case.
bash: prompt: fix breakage from $wd being gone
bash: prompt: fix ~/ special case when $wdtail is collapsed