Merge branch 'sg/test-split-index-fix'

Test updates. * sg/test-split-index-fix: read-cache: fix GIT_TEST_SPLIT_INDEX tests: disable GIT_TEST_SPLIT_INDEX for sparse index tests read-cache: look for shared index files next to the index, too t1600-index: disable GIT_TEST_SPLIT_INDEX t1600-index: don't run git commands upstream of a pipe t1600-index: remove unnecessary redirection