RFR: 2454: Skara mistakenly marked a non-clean backport as clean

[Zhao Song]

A user reported that Skara bot mistakenly marked a non-clean backport as clean. After investigation, I found that the PR was too large for the remote host to return the entire diff. The PR contained 8082 changed files, but when Skara bot requested the diff, only 3000 files were returned. Therefore, Skara bot was unable to compare some patches, so mistakenly mark the pr as clean.

In this patch, Skara bot will now check if the PR is too large to evaluate. If it is, Skara bot will not evaluate if this PR is clean and will add a warning comment to the PR instead.

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Commit messages:
 - update
 - update copyright
 - SKARA-2454

Changes: https://git.openjdk.org/skara/pull/1708/files
  Webrev: https://webrevs.openjdk.org/?repo=skara&pr=1708&range=00
  Issue: https://bugs.openjdk.org/browse/SKARA-2454
  Stats: 98 lines in 8 files changed: 90 ins; 0 del; 8 mod
  Patch: https://git.openjdk.org/skara/pull/1708.diff
  Fetch: git fetch https://git.openjdk.org/skara.git pull/1708/head:pull/1708

PR: https://git.openjdk.org/skara/pull/1708