RFR: 1332: Skara sometimes fails to detect that a backport was clean

[Zhao Song]

The pr bot sometimes fails to detect a backport is clean.

This issue for GitHub part was resolved by Guoxiong Li in [SKARA-1406](https://bugs.openjdk.org/browse/SKARA-1406).

This issue for GitLab part will be resolved in this patch.

Investigated some GitLab MR examples, I found that although the GitLab UI shows there are diffs in some files, when the bot queries changes of the pr, GitLab would tell the bot there are no diffs in some files. 

After reading the GitLab Api document, I found that we should set `access_raw_diffs` to `true` to get all the diffs.

https://docs.gitlab.com/ee/api/merge_requests.html#get-single-merge-request-changes

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Commit messages:
 - SKARA-1332

Changes: https://git.openjdk.org/skara/pull/1515/files
 Webrev: https://webrevs.openjdk.org/?repo=skara&pr=1515&range=00
  Issue: https://bugs.openjdk.org/browse/SKARA-1332
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/skara/pull/1515.diff
  Fetch: git fetch https://git.openjdk.org/skara.git pull/1515/head:pull/1515

PR: https://git.openjdk.org/skara/pull/1515