convert.c: remove an implicit dependency on the_index

convert.c: remove an implicit dependency on the_index

Make the convert API take an index_state instead of assuming the_index
in convert.c. All external call sites are converted blindly to keep
the patch simple and retain current behavior. Individual call sites
may receive further updates to use the right index instead of
the_index.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>