RFR: 1734: Reduce disk usage in pr bot [v2]

[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.

Zhao Song has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains six additional commits since the last revision:

 - fix import
 - fix import
 - Merge branch 'master' into SKARA-1734
   
   # Conflicts:
   #	bots/pr/src/main/java/org/openjdk/skara/bots/pr/CSRCommand.java
 - add some comments
 - introduce ScratchArea
 - SKARA-1734

-------------

Changes:
  - all: https://git.openjdk.org/skara/pull/1491/files
  - new: https://git.openjdk.org/skara/pull/1491/files/1c309904..970effcc

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=skara&pr=1491&range=01
 - incr: https://webrevs.openjdk.org/?repo=skara&pr=1491&range=00-01

  Stats: 4683 lines in 87 files changed: 2600 ins; 1742 del; 341 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