update-bash: don't prompt for passwords.

We execute too many `git fetch` at once for this to be useful. Just let
them fail instead and make it up to users to setup username/password
caching or SSH agents.

Library/Homebrew/cmd/update-bash.sh

@@ -303,6 +303,9 @@ EOS
       odie "Git must be installed and in your PATH!"
     fi
   fi
+  export GIT_TERMINAL_PROMPT="0"
+  export GIT_ASKPASS="false"
+  export GIT_SSH_COMMAND="ssh -oBatchMode=yes"
 
   if [[ -z "$HOMEBREW_VERBOSE" ]]
   then