RFR: 2418: Scratch dir clash in notifier

[Erik Joelsson]

The notifier RepositoryWorkItem and PullRequestWorkItem have a clash between directory location for the history repos. In RepositoryWorkItem we store the tags and branches history in these directories:


<scratch>/notify/history/tags
<scratch>/notify/history/branches


While in the PullRequestWorkItem, the history is stored in:


<scratch>/notify/history


Having repositories nested inside each other like this is causing inefficiencies at the very least, but could also potentially be harmful. As a quick fix, I propose we move the pr history to

 ```
 <scratch>/notify/history/pr
 ```

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

Commit messages:
 - SKARA-2418

Changes: https://git.openjdk.org/skara/pull/1700/files
  Webrev: https://webrevs.openjdk.org/?repo=skara&pr=1700&range=00
  Issue: https://bugs.openjdk.org/browse/SKARA-2418
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/skara/pull/1700.diff
  Fetch: git fetch https://git.openjdk.org/skara.git pull/1700/head:pull/1700

PR: https://git.openjdk.org/skara/pull/1700