RFR: 732: Large changes cause empty webrevs

[Erik Helin]

Hi all,

please review this patch that makes sure that we don't replace `commits.json`, `comparison.json` nor `metadata.json` with placeholders for webrevs.

Thanks,
Erik

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Commit messages:
 - skara-732

Changes: https://git.openjdk.java.net/skara/pull/977/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=977&range=00
  Issue: https://bugs.openjdk.java.net/browse/SKARA-732
  Stats: 7 lines in 1 file changed: 6 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/skara/pull/977.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/977/head:pull/977

PR: https://git.openjdk.java.net/skara/pull/977