[BUG] merge into unborn branch resets index+WT even if populated

Hi,

I'm posting this on behalf of Dscho who reported it on IRC, but apparently doesn't have the time for email.

If you run the following sequence of commands:

  git init
  echo foo > somefile
  git add somefile
  git pull elsewhere master

(where 'elsewhere' is any repo having a master branch) the "merge into unborn branch" logic silently nukes the index AND worktree copy of "somefile", thereby losing it. The same works with fetch+merge, so it's not pull's fault.