RFR: notify: store resulting commit in PullRequestState

[Erik Helin]

Hi all,

please review this patch that the resulting (integrated) commit from a PR in the `PullRequestState`. This data will be
used in future patches for more easier handling of notifications. Note that all data in the `Storage` might not have
the `"commit"` field. If that is the case, then the code will lazily populate the `"commit"` field.

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

Thanks,
Erik

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Commit messages:
 - notify: store resulting commit in PullRequestState

Changes: https://git.openjdk.java.net/skara/pull/641/files
 Webrev: https://webrevs.openjdk.java.net/skara/641/webrev.00
  Stats: 80 lines in 2 files changed: 57 ins; 0 del; 23 mod
  Patch: https://git.openjdk.java.net/skara/pull/641.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/641/head:pull/641

PR: https://git.openjdk.java.net/skara/pull/641