RFR: 1950: Don't run the second jcheck if there is no .jcheck/conf in source branch

[Zhao Song]

[SKARA-1937](https://bugs.openjdk.org/browse/SKARA-1937) triggered CheckWorkItem for many “ancient” prs and if the pr’s source branch doesn’t contain .jcheck/conf , it would trigger the second run of jcheck and then throw some exceptions.

In this patch, before the bot runs the second round of jcheck, it will check if .jcheck/conf exists in the "merge" branch.

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Commit messages:
 - SKARA-1950

Changes: https://git.openjdk.org/skara/pull/1533/files
 Webrev: https://webrevs.openjdk.org/?repo=skara&pr=1533&range=00
  Issue: https://bugs.openjdk.org/browse/SKARA-1950
  Stats: 7 lines in 2 files changed: 1 ins; 0 del; 6 mod
  Patch: https://git.openjdk.org/skara/pull/1533.diff
  Fetch: git fetch https://git.openjdk.org/skara.git pull/1533/head:pull/1533

PR: https://git.openjdk.org/skara/pull/1533