Stash apply: stage new files even when not updating the index

[ethomson]

Stash application should stage new files, even when we're not updating the index.

C:\Temp\stash_apply_sucks>git status
On branch master
Changes to be committed:
  (use "git reset HEAD <file>..." to unstage)

        new file:   bar
        modified:   foo


C:\Temp\stash_apply_sucks>git stash
Saved working directory and index state WIP on master: 4a6df6d foo
HEAD is now at 4a6df6d foo

C:\Temp\stash_apply_sucks>git status
On branch master
nothing to commit, working directory clean

C:\Temp\stash_apply_sucks>git stash apply
On branch master
Changes to be committed:
  (use "git reset HEAD <file>..." to unstage)

        new file:   bar

Changes not staged for commit:
  (use "git add <file>..." to update what will be committed)
  (use "git checkout -- <file>..." to discard changes in working directory)

        modified:   foo

WHAT A SENSIBLE ARCHITECTURE THIS IS. So we need to do the same thing. We break the actual iterator walking code out of merge to create a new index that has just the new files, which we will set as the new repo's index (unless the user actually specified that they wanted to restore the index).

[ethomson]

Oh, right, it turns out that we should also block stash application if there are any staged changes.