remove the worktree dir if it already exists

[Liujingfang1]

We saw the git-sync log errors like this

"Run(git worktree add /repo/root/rev-<COMMIT> origin/<BRANCH>): exit status 128:
 { stdout: \"Preparing worktree (detached HEAD 820c5bf)\\n\", 
stderr: \"fatal: '/repo/root/rev-<COMMIT>' already exists\\n\" 

The existing dir repo/root/rev-<COMMIT> was from a previous unsuccessful sync.

It blocks that syncing the latest commit. This can be fixed by removing the pre-existing dir before running git worktree add.