SourceTree hangs after clicking Stage Selected or Stage All on large files

Replication steps:

1. Prepare a git repo with some large files.
2. Stage all the files at once
3. Wonder if you clicked the "Stage selected" or "Stage All" button and click them again
4. Wait a bit, then get error about git process already in use and locked

Suggested solution:

1. After clicking stage or stage all, show a progress bar and grey out the stage all and stage selected buttons, just like what happens when commit is clicked
2. When the staging is completed, remove the progress bar.