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.
Merge branch 'master' into next

* master:
  Revert "Add new @ shortcut for HEAD"
Merge branch 'reset-submodule-trailing-slash' into jk/pu

* branch reset-submodule-trailing-slash:
  WIP built on nd/magic-pathspecs

Waiting for nd/magic-pathspecs to graduate to master.
Merge branch 'rebase-i-comment-squash' into jk/pu

* branch rebase-i-comment-squash:
  rebase -i: comment first line of autosquashed message

Comment out the first line of a "!squash" commit when generating a
commit message in git-rebase.

Merged to jk/pu on 2013-03-30.  Dogfooding for a while to see if
this is a sensible change.