Handle merge conflict before sending Commit

[bafu]

Currently, add will sync the latest Asset Tree and overlay the user's update on it.

In the time period between add to commit, if there is any new Commit, the user's staging Asset Tree / Commit will not be aware of it.

[bafu]

Hackathon summary

1. Paul's current idea: Record the block number of the latest Asset Tree in a temp file when add. When commit, check if the saved block number is still the latest one. If not, it means that there are updates happened between add and commit.