RFR: 1734: Reduce disk usage in pr bot

[Zhao Song]

As Erik mentioned in the issue, currently, pr bot would create multiple repository clones in different sub directory in the scratch directory which resulting in significant disk space wastage within the container. 

In this patch, the repository clones will be stored in a single directory at `scratch-x/pr/repos/repoName`, allowing different types of `WorkItems` to reuse this space.

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Commit messages:
 - SKARA-1734

Changes: https://git.openjdk.org/skara/pull/1491/files
 Webrev: https://webrevs.openjdk.org/?repo=skara&pr=1491&range=00
  Issue: https://bugs.openjdk.org/browse/SKARA-1734
  Stats: 10 lines in 4 files changed: 0 ins; 0 del; 10 mod
  Patch: https://git.openjdk.org/skara/pull/1491.diff
  Fetch: git fetch https://git.openjdk.org/skara.git pull/1491/head:pull/1491

PR: https://git.openjdk.org/skara/pull/1491