builtin-fast-export.c: handle nested tags

builtin-fast-export.c: handle nested tags

When tags that points to tags are passed to fast-export, an error is given,
saying "Tag [TAGNAME] points nowhere?". This fix calls parse_object() on the
object before referencing it's tag, to ensure the tag-info is fully initialized.
In addition, it inserts a comment to point out where nested tags are handled.
This is consistent with the comment for signed tags.

Signed-off-by: Erik Faye-Lund <kusmabite@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>