Use 'fast-forward' all over the place

It's a compound word.

Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/howto/update-hook-example.txt

@@ -76,7 +76,7 @@ case "$1" in
     if expr "$2" : '0*$' >/dev/null; then
       info "The branch '$1' is new..."
     else
-      # updating -- make sure it is a fast forward
+      # updating -- make sure it is a fast-forward
       mb=$(git-merge-base "$2" "$3")
       case "$mb,$2" in
         "$2,$mb") info "Update is fast-forward" ;;