unpack-trees: avoid array out-of-bounds error

The loop in warn_conflicted_path() that checks for the count of entries with the same path uses "i+count" for the array entry. However, the loop only verifies that the value of count is below the array size. Fix this by adding i to the condition. I hit this condition during a test of the in-tree sparse-checkout feature, so it is exercised by the end of the series. Signed-off-by: Derrick Stolee <dstolee@microsoft.com> [jc: readability fix] Signed-off-by: Junio C Hamano <gitster@pobox.com>