read-cache.c: remove 'const' from index_has_changes()

read-cache.c: remove 'const' from index_has_changes()

This function calls do_diff_cache() which eventually needs to set this
"istate" to unpack_options->src_index [1]. This is an unfortunate fact
that unpack_trees() _will_ update [2] src_index so we can't really pass a
const index_state there. Just remove 'const'.

[1] Right now diff_cache() in diff-lib.c assigns the_index to
    src_index. But the plan is to get rid of the_index, so it should
    be 'istate' from here that gets assigned to src_index.

[2] Some transient bits in the source index are touched. Optional
    extensions can also be removed. But other than that the source
    tree should still be valid.

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