mktag.c: improve verification of tagger field and tests

mktag.c: improve verification of tagger field and tests

Since nearly its birth, git's tags have included a "tagger" field which
describes the name of tagger, email of tagger, and date and time of tagging.
But, this field was only loosely tested by git-mktag. Provide some thorough
testing for this field and also ensure that the tag header is separated
from the tag body by an empty line to reduce the convenience of creating
a flawed tag.

Signed-off-by: Brandon Casey <casey@nrlssc.navy.mil>
Signed-off-by: Junio C Hamano <gitster@pobox.com>