cache-tree: use index state repository in trace2 calls

cache-tree: use index state repository in trace2 calls

trace2 calls in cache-tree.c use the global 'the_repository',
even though cache_tree_update() has access to an explicit
repository pointer via 'istate->repo'.

Using the global repository can result in incorrect trace2
output when multiple repository instances are in use, as
events may be attributed to the wrong repository.

Use 'istate->repo' in cache_tree_update() to ensure correct
repository attribution.

Other call sites are left unchanged as they do not have
access to a repository instance.

Signed-off-by: Jayesh Daga <jayeshdaga99@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>