drop casts from users EMPTY_TREE_SHA1_BIN

This macro already evaluates to the correct type, as it
casts the string literal to "unsigned char *" itself
(and callers who want the literal can use the _LITERAL
form).

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/diff.c

@@ -327,7 +327,7 @@ int cmd_diff(int argc, const char **argv, const char *prefix)
 				add_head_to_pending(&rev);
 				if (!rev.pending.nr) {
 					struct tree *tree;
-					tree = lookup_tree((const unsigned char*)EMPTY_TREE_SHA1_BIN);
+					tree = lookup_tree(EMPTY_TREE_SHA1_BIN);
 					add_pending_object(&rev, &tree->object, "HEAD");
 				}
 				break;

merge-recursive.c

@@ -1914,7 +1914,7 @@ int merge_recursive(struct merge_options *o,
 		/* if there is no common ancestor, use an empty tree */
 		struct tree *tree;
 
-		tree = lookup_tree((const unsigned char *)EMPTY_TREE_SHA1_BIN);
+		tree = lookup_tree(EMPTY_TREE_SHA1_BIN);
 		merged_common_ancestors = make_virtual_commit(tree, "ancestor");
 	}
 

sequencer.c

@@ -164,7 +164,7 @@ static void write_message(struct strbuf *msgbuf, const char *filename)
 
 static struct tree *empty_tree(void)
 {
-	return lookup_tree((const unsigned char *)EMPTY_TREE_SHA1_BIN);
+	return lookup_tree(EMPTY_TREE_SHA1_BIN);
 }
 
 static int error_dirty_index(struct replay_opts *opts)