Update git refs

1) Update remoteRefs whenever fetched.
2) Request remote_refs async on "Git Branch" action. Local copy is returned immediately.

fixes #2346 

@dinhviethoa PTAL thanks.

Yes. First time opening the branch dialog will show remote branches which are present locally. It will also trigger a `git fetch` in background, which will update the `remoteBranches`. A better solution might be to add a `refresh` or `update` button in the dialog. I don't think that is critical  as per now.