t1600-index: disable GIT_TEST_SPLIT_INDEX

Tests in 't1600-index.sh' check that various bogus index version values are recognized and an appropriate warning message is issued. GIT_TEST_SPLIT_INDEX=1 is supposed to trigger index splitting randomly, and thus might interfere [1] with these tests: splitting the index means that two index files are written (the shared base index and the split '.git/index'), and the same warning message is then issued twice, failing these tests. Unset GIT_TEST_SPLIT_INDEX in this test script to avoid such interference. [1] There is no such interference at the moment, because, alas, GIT_TEST_SPLIT_INDEX=1 is broken and never split the index. There was no such interference in the past (before it broke) either, because the first index write with GIT_TEST_SPLIT_INDEX=1 never split the index, only the second write did. A subsequent commit fixing GIT_TEST_SPLIT_INDEX will have all the details on this. Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com> Acked-by: Derrick Stolee <dstolee@microsoft.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>