update.sh: always fetch tags.

We use these for updating people who just follow tags.

Library/Homebrew/cmd/update.sh

@@ -480,10 +480,10 @@ EOS
 
       if [[ -n "$HOMEBREW_UPDATE_PREINSTALL" ]]
       then
-        git fetch --force "${QUIET_ARGS[@]}" origin \
+        git fetch --tags --force "${QUIET_ARGS[@]}" origin \
           "refs/heads/$UPSTREAM_BRANCH_DIR:refs/remotes/origin/$UPSTREAM_BRANCH_DIR" 2>/dev/null
       else
-        if ! git fetch --force "${QUIET_ARGS[@]}" origin \
+        if ! git fetch --tags --force "${QUIET_ARGS[@]}" origin \
           "refs/heads/$UPSTREAM_BRANCH_DIR:refs/remotes/origin/$UPSTREAM_BRANCH_DIR"
         then
           echo "Fetching $DIR failed!" >>"$update_failed_file"