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.

There are couple of reason I am skeptical doing it.

1) It may delay showing the dialogue, if somehow the network request becomes slow.
2) If a user is offline, the network request will fail. (though this can be handled in code).

I will prefer, not doing doing network-request before showing any actionDialog.