Use git-tag in git-cvsimport

Use git-tag in git-cvsimport

Currently git-cvsimport tries to create tag objects directly via git-mktag
in a very broken way, e.g the stuff it writes into the tagger field of
the tag object doesn't really resemble the GIT_COMMITTER_IDENT. This makes
gitweb and possibly other tools that try to interpret tag objects to be
confused about tag date and authorship.

Fix this by calling git-tag instead. This also has a nice side effect of
not creating the tag object but only the lightweight tag as that's the only
thing CVS has anyways.

Signed-off-by: Elvis Pranskevichus <el@prans.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>