RFR: 2190: Setting maximum size limit for PR diff in generating Webrev

[Zhao Song]

https://github.com/openjdk/tsan/pull/17, there are more than 5000 commits in this pr and mlbridge bot was trying to generate the webrev. The generation of webrev was successful but the webrev file was too big and exceeded GitHub's file size limit.

So as ErikJ suggested, if the diff of the pr is too large, we shouldn't generate webrev for it.

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Commit messages:
 - add test
 - update
 - SKARA-2190
 - update
 - update
 - update
 - SKARA-2190

Changes: https://git.openjdk.org/skara/pull/1617/files
 Webrev: https://webrevs.openjdk.org/?repo=skara&pr=1617&range=00
  Issue: https://bugs.openjdk.org/browse/SKARA-2190
  Stats: 232 lines in 9 files changed: 173 ins; 0 del; 59 mod
  Patch: https://git.openjdk.org/skara/pull/1617.diff
  Fetch: git fetch https://git.openjdk.org/skara.git pull/1617/head:pull/1617

PR: https://git.openjdk.org/skara/pull/1617