build: make code "-Wpointer-arith" clean

Recently introduced resolve_undo_read() expected arithmetic to (void *)
to work on byte-addresses.  Correct this.

Noticed by Brandon Casey.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

resolve-undo.c

@@ -53,7 +53,7 @@ void resolve_undo_write(struct strbuf *sb, struct string_list *resolve_undo)
 	for_each_string_list(write_one, resolve_undo, sb);
 }
 
-struct string_list *resolve_undo_read(void *data, unsigned long size)
+struct string_list *resolve_undo_read(const char *data, unsigned long size)
 {
 	struct string_list *resolve_undo;
 	size_t len;
@@ -93,7 +93,7 @@ struct string_list *resolve_undo_read(void *data, unsigned long size)
 				continue;
 			if (size < 20)
 				goto error;
-			hashcpy(ui->sha1[i], data);
+			hashcpy(ui->sha1[i], (const unsigned char *)data);
 			size -= 20;
 			data += 20;
 		}

resolve-undo.h

@@ -8,7 +8,7 @@ struct resolve_undo_info {
 
 extern void record_resolve_undo(struct index_state *, struct cache_entry *);
 extern void resolve_undo_write(struct strbuf *, struct string_list *);
-extern struct string_list *resolve_undo_read(void *, unsigned long);
+extern struct string_list *resolve_undo_read(const char *, unsigned long);
 extern void resolve_undo_clear_index(struct index_state *);
 extern int unmerge_index_entry_at(struct index_state *, int);
 extern void unmerge_index(struct index_state *, const char **);