Generate tags with correct timestamp (git-svnimport)

Now uses git-tag instead of manually constructing the tag.  This gives us a
correct timestamp, removes some crufty code, and makes it work the same as
git-cvsimport.

The generated tags are now lightweight tags instead of tag objects, which may
or may not be the behaviour we want.

Also, remove two unused variables from git-cvsimport.

Signed-off-by: Junio C Hamano <gitster@pobox.com>