RFR: 1950: Don't run the second jcheck if there is no .jcheck/conf in source branch [v2]

[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.

Zhao Song has updated the pull request incrementally with one additional commit since the last revision:

  update

-------------

Changes:
  - all: https://git.openjdk.org/skara/pull/1533/files
  - new: https://git.openjdk.org/skara/pull/1533/files/ae0e09d0..77a6602f

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=skara&pr=1533&range=01
 - incr: https://webrevs.openjdk.org/?repo=skara&pr=1533&range=00-01

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 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