t3000: add more testcases testing a variety of ls-files issues

t3000: add more testcases testing a variety of ls-files issues

This adds seven new ls-files tests.  While currently all seven test
pass, my earlier rounds of restructuring dir.c to replace an exponential
algorithm with a linear one passed all the tests in the testsuite but
failed six of these seven new tests.  Add these tests to increase our
case coverage.

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