merge: make restore_state() restore staged state too

merge: make restore_state() restore staged state too

There are multiple issues at play here:

  1) If `git merge` is invoked with staged changes, it should abort
     without doing any merging, and the user's working tree and index
     should be the same as before merge was invoked.
  2) Merge strategies are responsible for enforcing the index == HEAD
     requirement. (See 9822175d2b ("Ensure index matches head before
     invoking merge machinery, round N", 2019-08-17) for some history
     around this.)
  3) Merge strategies can bail saying they are not an appropriate
     handler for the merge in question (possibly allowing other
     strategies to be used instead).
  4) Merge strategies can make changes to the index and working tree,
     and have no expectation to clean up after themselves, *even* if
     they bail out and say they are not an appropriate handler for
     the merge in question.  (The `octopus` merge strategy does this,
     for example.)
  5) Because of (3) and (4), builtin/merge.c stashes state before
     trying merge strategies and restores it afterward.

Unfortunately, if users had staged changes before calling `git merge`,
builtin/merge.c could do the following:

   * stash the changes, in order to clean up after the strategies
   * try all the merge strategies in turn, each of which report they
     cannot function due to the index not matching HEAD
   * restore the changes via "git stash apply"

But that last step would have the net effect of unstaging the user's
changes.  Fix this by adding the "--index" option to "git stash apply".
While at it, also squelch the stash apply output; we already report
"Rewinding the tree to pristine..." and don't need a detailed `git
status` report afterwards.  Also while at it, switch to using strvec
so folks don't have to count the arguments to ensure we avoided an
off-by-one error, and so it's easier to add additional arguments to
the command.

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