index: improve constness for reading blob data

Improve constness of the index_state parameter to the
'read_blob_data_from_index' function.
Signed-off-by: NBrandon Williams <bmwill@google.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

cache.h

@@ -599,7 +599,7 @@ extern int chmod_index_entry(struct index_state *, struct cache_entry *ce, char
 extern int ce_same_name(const struct cache_entry *a, const struct cache_entry *b);
 extern void set_object_name_for_intent_to_add_entry(struct cache_entry *ce);
 extern int index_name_is_other(const struct index_state *, const char *, int);
-extern void *read_blob_data_from_index(struct index_state *, const char *, unsigned long *);
+extern void *read_blob_data_from_index(const struct index_state *, const char *, unsigned long *);
 
 /* do stat comparison even if CE_VALID is true */
 #define CE_MATCH_IGNORE_VALID		01

read-cache.c

@@ -2285,7 +2285,8 @@ int index_name_is_other(const struct index_state *istate, const char *name,
 	return 1;
 }
 
-void *read_blob_data_from_index(struct index_state *istate, const char *path, unsigned long *size)
+void *read_blob_data_from_index(const struct index_state *istate,
+				const char *path, unsigned long *size)
 {
 	int pos, len;
 	unsigned long sz;