textutils 2.[01]: "fmt --prefix=TAB" doesn't work

fmt does not allow prefix strings starting with tab characters.
I know that tabs are special in that they are expanded on input,
but it doesn't look like that's the reason why this is broken
because prefix strings may contain tabs, provided the prefix
string doesn't start with one.

Steps to reproduce:

  $ fmt --prefix=TAB <<EOF
  TABfoo
  TABbar
  EOF

The correct output should be

  TABfoo bar

but fmt returns

  foo
  bar

-- 
 Haakon