Merge branch 'aw/numbered-stash'

The user always has to say "stash@{$N}" when naming a single
element in the default location of the stash, i.e. reflogs in
refs/stash.  The "git stash" command learned to accept "git stash
apply 4" as a short-hand for "git stash apply stash@{4}".

* aw/numbered-stash:
  stash: allow stashes to be referenced by index only