RFR: 1512: MirrorBot may get stuck failing to clone repository

[Zhao Song]

Optimized the logic in MirrorBot.

When MirrorBot tries to create a directory, the directory will have a 'temporary' suffix, then it will attempts cloning. It the cloning succeed, the suffix will be removed. If the clone fails or the bot happens to die, the directory will retain the suffix. On the next time, if we try to rerun the bot, the bot will detect the temporary directory and will delete the temporary directory first, therefore, the bot will not get stuck.

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Commit messages:
 - In MirrorBot, just falling back on recloning when existing scratch directory has problem
 - optimized MirrorBot

Changes: https://git.openjdk.org/skara/pull/1374/files
 Webrev: https://webrevs.openjdk.org/?repo=skara&pr=1374&range=00
  Issue: https://bugs.openjdk.org/browse/SKARA-1512
  Stats: 55 lines in 2 files changed: 52 ins; 1 del; 2 mod
  Patch: https://git.openjdk.org/skara/pull/1374.diff
  Fetch: git fetch https://git.openjdk.org/skara pull/1374/head:pull/1374

PR: https://git.openjdk.org/skara/pull/1374