RFR: webrev: follow files for commits.json

[Erik Helin]

Hi all,

please review this patch that updates `Webrev.generateJSON` to use `ReadOnlyRepository.follow` for getting commit
metadata.

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

Thanks,
Erik

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Commit messages:
 - webrev: follow files for commits.json

Changes: https://git.openjdk.java.net/skara/pull/741/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=741&range=00
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/skara/pull/741.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/741/head:pull/741

PR: https://git.openjdk.java.net/skara/pull/741