The tests for {reset,commit,stash} -p will frequently have to set both
worktree and index states to known values, and verify that the outcome
(again both worktree and index) are what was expected.
Add a small helper library that lets us do these tasks more easily.
Signed-off-by: Thomas Rast <trast@student.ethz.ch>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
