fast-export: deal with tag objects that do not have a tagger

fast-export: deal with tag objects that do not have a tagger

When no tagger was found (old Git produced tags like this),
no "tagger" line is printed (but this is incompatible with the current
git fast-import).

Alternatively, you can pass the option --fake-missing-tagger, forcing
fast-export to fake a tagger

	Unspecified Tagger <no-tagger>

with a tag date of the beginning of (Unix) time in the case of a missing
tagger, so that fast-import is still able to import the result.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>