RFR: Save references to materialized branches locally

[Robin Westberg]

Hi all,

Please review this change that ensure that when bots use `Repository.materialize`, they always save a named reference to what was fetched. This ensures that subsequent fetches will present these existing refs, and avoid re-fetching commits that already exist locally.

It also fixes a potential bug where the mlbridge archive reader could have fetched from an incorrect branch.

Best regards,
Robin

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Commits:
 - 280c73b8: Use configurable ref for the mlbridge archive
 - 6b707ae4: Specify a destination ref when using materialize

Changes: https://git.openjdk.java.net/skara/pull/271/files
 Webrev: https://webrevs.openjdk.java.net/skara/271/webrev.00
  Stats: 54 lines in 14 files changed: 28 ins; 0 del; 26 mod
  Patch: https://git.openjdk.java.net/skara/pull/271.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/271/head:pull/271

PR: https://git.openjdk.java.net/skara/pull/271