fast-import: don't allow to tag empty branch

fast-import: don't allow to tag empty branch

'reset' command makes fast-import start a branch from scratch. It's name
is kept in lookup table but it's sha1 is null_sha1 (special value).
'tag' command can be used to tag a branch by it's name. lookup_branch()
is used it that case and it doesn't check for null_sha1. So fast-import
writes a tag for null_sha1 object instead of giving a error.

Add a check to deny tagging an empty branch and add a corresponding test.

Signed-off-by: Dmitry Ivankov <divanorama@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>