test-dump-cache-tree: avoid overflow of cache-tree name

test-dump-cache-tree: avoid overflow of cache-tree name

When dumping a cache-tree, we sprintf sub-tree names directly
into a fixed-size buffer, which can overflow. We can
trivially fix this by converting to xsnprintf to at least
notice and die.

This probably should handle arbitrary-sized names, but
there's not much point. It's used only by the test scripts,
so the trivial fix is enough.

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