t1092: test 'git restore' with sparse index

t1092: test 'git restore' with sparse index

A user reported that 'git restore --staged .' causes the sparse index to
expand. This is somewhat natural because the '.' pathspec means 'check
every path'. However, the restore will not update paths marked with the
SKIP_WORKTREE bit, so we shouldn't need to process such entries.

For now, establish the current behavior, including the sparse index
expansion, in the t1092 test case as a baseline.

Signed-off-by: Derrick Stolee <stolee@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>