add -p: import Term::ReadKey with 'require'

add -p: import Term::ReadKey with 'require'

eval{use...} is no good because the 'use' is evaluated at compile
time, so manually 'require' it.  We need to forward declare the
functions we use, otherwise Perl raises a compilation error.

Signed-off-by: Thomas Rast <trast@student.ethz.ch>
Signed-off-by: Junio C Hamano <gitster@pobox.com>