file-server: fix base-hash trimming
Fixes
*@uv
=/ parent (scot %p ship.u.ota)
=+ .^(=cass:clay %cs /[parent]/[desk.u.ota]/1/late/foo)
- %^ end 3 3
+ %^ end 0 25
.^(@uv %cz /[parent]/[desk.u.ota]/(scot %ud ud.cass))
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