Revert "git stash: avoid data loss when "git stash save" kills a directory"

This reverts commit a73653130edd6a8977106d45a8092c09040f9132, as it
has been reported that "ls-files --killed" is too time-consuming in
a deep directory with too many untracked crufts (e.g. $HOME/.git
tracking only a few files).

We'd need to revisit it later but "ls-files --killed" needs to be
optimized before it happens.
Revert "Add new @ shortcut for HEAD"

This reverts commit cdfd94837b27c220f70f032b596ea993d195488f, as it
does not just apply to "@" (and forms with modifiers like @{u}
applied to it), but also affects e.g. "refs/heads/@/foo", which it
shouldn't.

The basic idea of giving a short-hand might be good, and the topic
can be retried later, but let's revert to avoid affecting existing
use cases for now for the upcoming release.