reset: add test cases for "--keep" option

reset: add test cases for "--keep" option

This shows that with the "--keep" option, changes that are both in
the work tree and the index are kept in the work tree after the
reset (but discarded in the index).

In the case of unmerged entries, we can see that "git reset --keep"
works only when the target state is the same as HEAD. And then the
work tree is not reset.

Signed-off-by: Christian Couder <redacted>
Signed-off-by: Junio C Hamano <redacted>