RFR: Retry updating seed if needed, avoid unnecessary fetching

[Robin Westberg]

Retry fetching a few times if a seed is not allowed to be stale, but otherwise ignore and continue.

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Commit messages:
 - Retry updating seed if needed, avoid unnecessary fetching

Changes: https://git.openjdk.java.net/skara/pull/902/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=902&range=00
  Stats: 41 lines in 1 file changed: 24 ins; 3 del; 14 mod
  Patch: https://git.openjdk.java.net/skara/pull/902.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/902/head:pull/902

PR: https://git.openjdk.java.net/skara/pull/902