RFR: git-sync: use upstream remote and support fork remote

[Robin Westberg]

On Mon, 9 Dec 2019 10:08:28 GMT, Erik Helin <ehelin at openjdk.org> wrote:

> Hi all,
> 
> please review this patch that makes `git-sync` use the `upstream` remote if it is available. `git-sync` has also learned that if the `fork` remote is present, then it should sync from `origin` to `fork`.
> 
> I also updated the names to make them clearer, the words `upstream` and `origin` were confusing.
> 
> Thanks,
> Erik
> 
> ## Testing
> - [x] Manual testing of `git-sync` on Linux x64

Looks good!

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Marked as reviewed by rwestberg (Reviewer).

PR: https://git.openjdk.java.net/skara/pull/301