update.sh: further tweak update fix.

Library/Homebrew/cmd/update.sh

@@ -479,11 +479,13 @@ EOS
 
     PREFETCH_REVISION_VAR="PREFETCH_REVISION$TAP_VAR"
     PREFETCH_REVISION="${!PREFETCH_REVISION_VAR}"
+    POSTFETCH_REVISION="$(git rev-parse -q --verify refs/remotes/origin/"$UPSTREAM_BRANCH")"
 
     if [[ -n "$HOMEBREW_SIMULATE_FROM_CURRENT_BRANCH" ]]
     then
       simulate_from_current_branch "$DIR" "$TAP_VAR" "$UPSTREAM_BRANCH" "$CURRENT_REVISION"
-    elif [[ "$PREFETCH_REVISION" = "$CURRENT_REVISION" ]]
+    elif [[ "$PREFETCH_REVISION" = "$POSTFETCH_REVISION" ]] &&
+         [[ "$CURRENT_REVISION" = "$POSTFETCH_REVISION" ]]
     then
       export HOMEBREW_UPDATE_BEFORE"$TAP_VAR"="$CURRENT_REVISION"
       export HOMEBREW_UPDATE_AFTER"$TAP_VAR"="$CURRENT_REVISION"