tree: move layout into page function

This also allows us to return proper HTTP error codes when the requested
tree is not found and display an error message in one case (invalid path
inside valid commit) where we previously just displayed an empty page.

Signed-off-by: John Keeping <john@keeping.me.uk>

cmd.c

@@ -172,7 +172,7 @@ struct cgit_cmd *cgit_get_cmd(void)
 		def_cmd(stats, 1, 0, 1, 0),
 		def_cmd(summary, 1, 0, 0, 0),
 		def_cmd(tag, 1, 0, 0, 0),
-		def_cmd(tree, 1, 1, 1, 0),
+		def_cmd(tree, 1, 0, 1, 0),
 	};
 	int i;