update.sh: allow forcing tag update.

To test the tag update functionality allow setting
`HOMEBREW_UPDATE_TO_TAG`.

Library/Homebrew/brew.sh

-HOMEBREW_VERSION="0.9.9"
+HOMEBREW_VERSION="1.0.0"
 
 onoe() {
   if [[ -t 2 ]] # check whether stderr is a tty.

Library/Homebrew/cmd/update.sh

@@ -348,7 +348,7 @@ EOS
     set -x
   fi
 
-  if [[ -z "$HOMEBREW_UPDATE_CLEANUP" ]]
+  if [[ -z "$HOMEBREW_UPDATE_CLEANUP" && -z "$HOMEBREW_UPDATE_TO_TAG" ]]
   then
     if [[ -n "$HOMEBREW_DEVELOPER" || -n "$HOMEBREW_DEV_CMD_RUN" ]]
     then