RFR: 2176: Repository.fetch should return an Optional<Hash>

[Erik Duveblad]

Hi all,

please review this patch that makes `Repository.fetch` return an `Optional<Hash>` instead of just a `Hash` (since a `fetch` operation might now always fetch a commit, for example if code is trying to fetch a ref that does not exist in the remote repo). A fairly large diff, but I just moved the `.orElseThrow` one frame up in the call chain, so now the caller decides whether it should do `.orElseThrow` (instread of the callee, in this case `fetch`).

Thanks,
Erik

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Commit messages:
 - 2176

Changes: https://git.openjdk.org/skara/pull/1613/files
 Webrev: https://webrevs.openjdk.org/?repo=skara&pr=1613&range=00
  Issue: https://bugs.openjdk.org/browse/SKARA-2176
  Stats: 85 lines in 37 files changed: 0 ins; 3 del; 82 mod
  Patch: https://git.openjdk.org/skara/pull/1613.diff
  Fetch: git fetch https://git.openjdk.org/skara.git pull/1613/head:pull/1613

PR: https://git.openjdk.org/skara/pull/1613