read-cache: use istate->repo for trace2 logging

read-cache: use istate->repo for trace2 logging

trace2 calls in read-cache.c use the global 'the_repository',
even though the relevant index_state provides 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' instead to ensure correct repository
attribution.

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