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

cache.h

@@ -703,7 +703,7 @@ extern int unmerged_index(const struct index_state *);
  * provided, the space-separated list of files that differ will be appended
  * to it.
  */
-extern int index_has_changes(const struct index_state *istate,
+extern int index_has_changes(struct index_state *istate,
 			     struct tree *tree,
 			     struct strbuf *sb);
 

read-cache.c

@@ -2122,7 +2122,7 @@ int unmerged_index(const struct index_state *istate)
 	return 0;
 }
 
-int index_has_changes(const struct index_state *istate,
+int index_has_changes(struct index_state *istate,
 		      struct tree *tree,
 		      struct strbuf *sb)
 {