RFR: Ensure that seeded checkouts can be preserved

[Robin Westberg]

Preserving a seeded clone that fails checkout should not move files across volume boundaries. Also ensure that seeds
are only updated periodically.

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Commit messages:
 - Always fetch conditionally
 - Repositories meant to be kept for later debugging should not be moved across volume boundaries

Changes: https://git.openjdk.java.net/skara/pull/877/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=877&range=00
  Stats: 91 lines in 2 files changed: 78 ins; 6 del; 7 mod
  Patch: https://git.openjdk.java.net/skara/pull/877.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/877/head:pull/877

PR: https://git.openjdk.java.net/skara/pull/877