t: add tests for cloning remotes with detached HEAD

t: add tests for cloning remotes with detached HEAD

We didn't test this setup at all, and doing so reveals a few
bugs:

  1. Cloning a repository with an orphaned detached HEAD
     (i.e., one that points to history that is not
     referenced by any ref) will fail.

  2. Cloning a repository with a detached HEAD that points
     to a tag will cause us to write a bogus "refs/tags/..."
     ref into the HEAD symbolic ref. We should probably
     detach instead.

  3. Cloning a repository with a detached HEAD that points
     to a branch will cause us to checkout that branch. This
     is a known limitation of the git protocol (we have to
     guess at HEAD's destination, since the symref contents
     aren't shown to us). This test serves to document the
     desired behavior, which can only be achieved once the
     git protocol learns to share symref information.

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