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.

On second thoughts, The Git Api is actually providing two APIs. One to `updateAndGet` and the second to `getLocal`. The caching is at the `scmLayer` and we use similar for `GitStatus`. I will prefer to leave it like this, until we explicitly need two apis.