This is related to issue1572.  It's likely that the same fix for issue1572 will
cover this as well, ie. a notion of pristine prefs.

. ./lib                         # Load some portability helpers.
rm -rf R                        # Another script may have left a mess.
darcs init      --repo R        # Create our test repos.
cd R
darcs setpref boringfile f
darcs record -am 'setpref'
rm _darcs/prefs/prefs
darcs repair
test -e _darcs/prefs/prefs

See issue1572 for a simple workaround.