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

[Erik Helin]

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

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Commits:
 - 9fb50503: git-sync: use upstream remote and support fork remote

Changes: https://git.openjdk.java.net/skara/pull/301/files
 Webrev: https://webrevs.openjdk.java.net/skara/301/webrev.00
  Stats: 38 lines in 1 file changed: 15 ins; 2 del; 21 mod
  Patch: https://git.openjdk.java.net/skara/pull/301.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/301/head:pull/301

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