revision: tolerate promised targets of tags

revision: tolerate promised targets of tags

In handle_commit(), it is fatal for an annotated tag to point to a
non-existent object. --exclude-promisor-objects should relax this rule
and allow non-existent objects that are promisor objects, but this is
not the case. Update handle_commit() to tolerate this situation.

This was observed when cloning from a repository with an annotated tag
pointing to a blob. The test included in this patch demonstrates this
case.

Signed-off-by: Jonathan Tan <jonathantanmy@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>