t/annotate-tests: Use echo & cat instead of sed

t/annotate-tests: Use echo & cat instead of sed

The use of the sed command "1i No robots allowed" caused the version
of sed in OS X to die with

   sed: 1: "1i "No robots allowed"\n": command i expects \ followed by
   text

Since this command was just trying to add a single line to the
beginning of the file, do the same with "echo" followed by "cat".

Unbreaks t8001 and t8002 on OS X 10.6.7

Signed-off-by: Brian Gernhardt <brian@gernhardtsoftware.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>