RFR: 1431: The method TestPullRequest#diff sometimes returns wrong information

[Guoxiong Li]

Hi all,

The method `TestPullRequest#diff` always passes the head hash of the target branch to the method `Repository#diff` as the base hash. But it may be wrong when the target branch was submitted some new commits after creating the pull request. `TestPullRequest#diff` should find the last common hash of the source and target branch as the base hash.

This patch fixes the bug and adds the test case. I don't know whether it is good to place the test case at the class `CSRBotTests`, but the method `CSRBotTests#testBackportCsr` indeed contains the situation of this bug.

Thanks for taking the time to review.

Best Regards,
-- Gxuoxiong

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Commit messages:
 - SKARA-1431

Changes: https://git.openjdk.org/skara/pull/1360/files
 Webrev: https://webrevs.openjdk.org/?repo=skara&pr=1360&range=00
  Issue: https://bugs.openjdk.org/browse/SKARA-1431
  Stats: 25 lines in 2 files changed: 17 ins; 3 del; 5 mod
  Patch: https://git.openjdk.org/skara/pull/1360.diff
  Fetch: git fetch https://git.openjdk.org/skara pull/1360/head:pull/1360

PR: https://git.openjdk.org/skara/pull/1360