I've got a relatively old darcs-1 format repo. It still has a _darcs/current
instead of the newer _darcs/pristine.

It seems that while some parts of darcs are quite happy with this old format
(including darcs check and repair), darcs whatsnew is not:

$ darcs whatsnew
darcs: ./_darcs/pristine: changeWorkingDirectory: does not exist (No such file
or directory)

It should be fairly straightforward to reproduce this. Just find an old repo
using _darcs/current.

As a workaround, it appears that this works:

$ mv _darcs/current _darcs/pristine

This is all using darcs version 2.3.0 (release)

If darcs is intending to stop supporting the old _darcs/current style then it'd
be good if darcs repair could update to the later darcs-1 layout.

This is a bug in the hashed-storage package to which some darcs functionality
has been outsourced starting with the 2.3 release. You should be able to solve
the problem by upgrading to hashed-storage 0.3.7.