git tag: don't complain of empty messages

Hello,

GIT refuses to set annotated tags without a comment. I think this is a 
silly restriction when the message is explicitly set to the empty string.

The attached patch should fix this;  Unfortunately, I've been unable to 
test it. Running the script with sh -x stops at

++exec /home/hanwen/usr/pkg/git/bin/git-sh-setup

-- 
  Han-Wen Nienhuys - hanwen@xs4all.nl - http://www.xs4all.nl/~hanwen