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>
diff --git a/t/t9301-fast-export.sh b/t/t9301-fast-export.sh
index b1f75ce..4a87f36 100755
--- a/t/t9301-fast-export.sh
+++ b/t/t9301-fast-export.sh
@@ -271,7 +271,7 @@
 # NEEDSWORK: not just check return status, but validate the output
 test_expect_success 'tree_tag'        'git fast-export tree_tag'
 test_expect_success 'tree_tag-obj'    'git fast-export tree_tag-obj'
-test_expect_failure 'tag-obj_tag'     'git fast-export tag-obj_tag'
-test_expect_failure 'tag-obj_tag-obj' 'git fast-export tag-obj_tag-obj'
+test_expect_success 'tag-obj_tag'     'git fast-export tag-obj_tag'
+test_expect_success 'tag-obj_tag-obj' 'git fast-export tag-obj_tag-obj'
 
 test_done