symbolic ref: refuse non-ref targets in HEAD

When calling "git symbolic-ref" it is easy to forget that
the target must be a fully qualified ref. E.g., you might
accidentally do:

  $ git symbolic-ref HEAD master

Unfortunately, this is very difficult to recover from,
because the bogus contents of HEAD make git believe we are
no longer in a git repository (as is_git_dir explicitly
checks for "^refs/heads/" in the HEAD target). So
immediately trying to fix the situation doesn't work:

  $ git symbolic-ref HEAD refs/heads/master
  fatal: Not a git repository

and one is left editing the .git/HEAD file manually.

Furthermore, one might be tempted to use symbolic-ref to set
up a detached HEAD:

  $ git symbolic-ref HEAD `git rev-parse HEAD`

which sets up an even more bogus HEAD:

  $ cat .git/HEAD
  ref: 1a9ace4f2ad4176148e61b5a85cd63d5604aac6d

This patch introduces a small safety valve to prevent the
specific case of anything not starting with refs/heads/ to
go into HEAD. The scope of the safety valve is intentionally
very limited, to make sure that we are not preventing any
behavior that would otherwise be valid (like pointing a
different symref than HEAD outside of refs/heads/).

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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