unpack-trees: don't shadow global var the_index

unpack-trees: don't shadow global var the_index

This function mark_new_skip_worktree() has an argument named the_index
which is also the name of a global variable. While they have different
types (the global the_index is not a pointer) mistakes can easily
happen and it's also confusing for readers. Rename the function
argument to something other than the_index.

Signed-off-by: Nguyễn Thái Ngọc Duy <redacted>
Signed-off-by: Junio C Hamano <redacted>