stash: honor stash.index in apply, pop modes

stash: honor stash.index in apply, pop modes

With stash.index=true, git-stash(1) command now tries to reinstate the
index by default in the "apply" and "pop" modes. Not doing so creates a
common trap [1], [2]: "git stash apply" is not the reverse of "git stash
push" because carefully staged indices are lost and have to be manually
recreated. OTOH, this mode is not always desirable and may create more
conflicts when applying stashes. As usual, "--no-index" will disable
this behavior if you set "stash.index".

[1]: https://lore.kernel.org/git/CAPx1GvcxyDDQmCssMjEnt6JoV6qPc5ZUpgPLX3mpUC_4PNYA1w@mail.gmail.com/
[2]: https://lore.kernel.org/git/c5a811ac-8cd3-c389-ac6d-29020a648c87@gmail.com/

Signed-off-by: D. Ben Knoble <ben.knoble+github@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>