subtree: use named variables instead of "$@" in cmd_pull

subtree: use named variables instead of "$@" in cmd_pull

'cmd_pull' already checks that only two arguments are given,
'repository' and 'ref'. Define variables with these names instead of
using the positional parameter $2 and "$@".

This will allow a subsequent commit to pass 'repository' to 'cmd_merge'.

Signed-off-by: Philippe Blain <levraiphilippeblain@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>