describe: catch unborn branch in describe_blob()

When describing a blob, we search for it by traversing from HEAD. We do
this by feeding the name HEAD to setup_revisions(). But if we are on an
unborn branch, this will fail with a confusing message:

  $ git describe $blob
  fatal: ambiguous argument 'HEAD': unknown revision or path not in the working tree.
  Use '--' to separate paths from revisions, like this:
  'git <command> [<revision>...] -- [<file>...]'

It is OK for this to be an error (we cannot find $blob in an empty
traversal, so we'd eventually complain about that). But the error
message could be more helpful.

Let's resolve HEAD ourselves and pass the resolved object id to
setup_revisions(). If resolving fails, then we can print a more useful
message.

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

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