RFR: Avoid resetting an existing storage file

[Robin Westberg]

If a materialization of a HostedRepositoryStorage fails due to an intermittent problem, we must ensure that we do not incorrectly try to create the ref as if it did not yet exist.

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Commit messages:
 - Avoid resetting an existing storage file

Changes: https://git.openjdk.java.net/skara/pull/933/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=933&range=00
  Stats: 14 lines in 2 files changed: 13 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/skara/pull/933.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/933/head:pull/933

PR: https://git.openjdk.java.net/skara/pull/933