t0090: stop losing return codes of git commands

t0090: stop losing return codes of git commands

In generate_expected_cache_tree_rec(), there are currently two instances
of `git ls-files` in the upstream of a pipe. In the case where the
upstream git command fails, its return code will be lost. Extract the
`git ls-files` into its own call so that if it ever fails, its return
code is not lost.

Signed-off-by: Denton Liu <liu.denton@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>