fast-import: tag may point to any object type

fast-import: tag may point to any object type

If you tried to export the official git repository, and then to import it
back then git-fast-import would die complaining that "Mark :1 not a commit".

Accordingly to a generated crash file, Mark 1 is not a commit but a blob,
which is pointed by junio-gpg-pub tag. Because git-tag allows to create such
tags, git-fast-import should import them.

Signed-off-by: Dmitry Potapov <redacted>
Acked-by: Shawn O. Pearce <redacted>
Signed-off-by: Junio C Hamano <redacted>