Integrated: pr: use separate scratch for commit-comments work item

[Erik Helin]

Hi all,

please review this patch puts the materialized repositories for the `CommitCommensWorkItem` into a separate directory.

Testing:
- [x] `make test` passes on Linux x64

Thanks,
Erik

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

Commit messages:
 - pr: use separate scratch for commit-comments work item

Changes: https://git.openjdk.java.net/skara/pull/1036/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=1036&range=00
  Stats: 3 lines in 1 file changed: 0 ins; 1 del; 2 mod
  Patch: https://git.openjdk.java.net/skara/pull/1036.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/1036/head:pull/1036

PR: https://git.openjdk.java.net/skara/pull/1036