The tags are listed in alphanumeric order, but some clients may wish
to order the tags by the date they were created.
Add a timestamp in the tag info.
For annotated/signed tags, the timestamp is the date at which the tag
was created.
For lightweight tags, the timestamp is set to the committer timestamp,
only when the tag is on a commit.
Change-Id: I34f49f19810fa430769a50b5ae43b8107527792b
c4e4d93410