RFR: git-sync should use APIs to find upstream

[Erik Helin]

Hi all,

this patch changes `git-sync` to to use the `Forge` APIs for finding the upstream repository for a personal fork (in case the `upstream` remote is missing). The big part of this change is making `Forge.repository` return `Optional<HostedRepository>` instead of `HostedRepository` (because a forge may not have the requested repository). Robin, what do you think about these changes?

Thanks,
Erik

## Testing
- [x] Manual testing of `git sync`, both with and without an `upstream` remote

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Commits:
 - 20e4e200: git-sync should use APIs to find upstream

Changes: https://git.openjdk.java.net/skara/pull/262/files
 Webrev: https://webrevs.openjdk.java.net/skara/262/webrev.00
  Stats: 70 lines in 15 files changed: 44 ins; 0 del; 26 mod
  Patch: https://git.openjdk.java.net/skara/pull/262.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/262/head:pull/262

PR: https://git.openjdk.java.net/skara/pull/262