subtree: define a variable before its first use in 'find_latest_squash'

subtree: define a variable before its first use in 'find_latest_squash'

The function 'find_latest_squash' takes a single argument, 'dir', but a
debug statement uses this variable before it takes its value from $1.

This statement thus gets the value of 'dir' from the calling function,
which currently is the same as the 'dir' argument, so it works but it
is confusing.

Move the definition of 'dir' before its first use.

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