Run the async_run and disown in a subshell to suppress Done output

gitprompt.sh

@@ -110,8 +110,10 @@ function checkUpstream() {
   if [[ ! -e "${FETCH_HEAD}"  ||  -e `find "${FETCH_HEAD}" -mmin +${GIT_PROMPT_FETCH_TIMEOUT}` ]]
   then
     if [[ -n $(git remote show) ]]; then
+      (
         async_run "git fetch --quiet"
         disown -h
+      )
     fi
   fi
 }