tag.c: Parse tagger date (if present)

tag.c: Parse tagger date (if present)

Just like with committer dates, we parse the tagger date into the
struct tag so its available for further downstream processing.
However since the tagger header was not introduced until Git 0.99.1
we must consider it optional.  For tags missing this header we use
the default date of 0.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>