t3507: add a testcase showing failure with sparse checkout

t3507: add a testcase showing failure with sparse checkout

Recent changes in merge_content() induced a bug when merging files that are
not present in the local working directory due to sparse-checkout. Add a
test case to demonstrate the bug so that we can ensure the fix resolves
it and to prevent future regressions.

Signed-off-by: Ben Peart <benpeart@microsoft.com>
Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>