jenkins job that executes benchmarks assumes commits are in scala/scala

[retronym]

Even though compiler-benchq can build commits from other users repos, execution in https://scala-ci.typesafe.com/view/scala-bench/job/compiler-benchmark currently fails with

[warn] one warning found
Processing 9 classes from /home/jenkins/workspace/compiler-benchmark/compilation/target/scala-2.12/classes with "reflection" generator
Writing out Java source to /home/jenkins/workspace/compiler-benchmark/compilation/target/scala-2.12/src_managed/jmh and resources to /home/jenkins/workspace/compiler-benchmark/compilation/target/scala-2.12/resource_managed/jmh
[info] Compiling 20 Java sources to /home/jenkins/workspace/compiler-benchmark/compilation/target/scala-2.12/classes...
[info] Running scala.bench.UploadingRunner HotScalacBenchmark -foe true -p source=better-files -wi 10 -w 10 -i 10 -r 10 -f 3
[error] Exception in thread "main" java.lang.IllegalArgumentException: Cannot resolve: df9813c from Repository[/home/jenkins/code/scala/.git]
[error] 	at scala.bench.GitWalker.resolve(GitWalker.java:126)
[error] 	at scala.bench.GitWalker.isAncestor(GitWalker.java:162)
[error] 	at scala.bench.ScalacBenchmarkRunner.corpusVersion(ScalacBenchmarkRunner.java:37)
[error] 	at scala.bench.ScalacBenchmarkRunner.setCorpusVersion(ScalacBenchmarkRunner.java:56)
[error] 	at scala.bench.UploadingRunner.main(UploadingRunner.java:18)
java.lang.RuntimeException: Nonzero exit code returned from runner: 1
	at scala.sys.package$.error(package.scala:27)

The workaround is to submit a PR to scala/scala with the WIP branch.

We should probably add a parameter to this build with the repo and sha, and add a step to the script to fetch that commit.