RFR: Standardize pr bot scratch folder names and detect invalid directories

[Robin Westberg]

Hi all,

Please review this small change that ensures that the PR bot uses unique folders for scratch repositories, and removes invalid destination directories before attempting a fresh clone.

Best regards,
Robin

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Commits:
 - 8b5e1d1e: Standardize pr bot scratch folder names and detect invalid directories as clone targets

Changes: https://git.openjdk.java.net/skara/pull/424/files
 Webrev: https://webrevs.openjdk.java.net/skara/424/webrev.00
  Stats: 18 lines in 6 files changed: 3 ins; 0 del; 15 mod
  Patch: https://git.openjdk.java.net/skara/pull/424.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/424/head:pull/424

PR: https://git.openjdk.java.net/skara/pull/424