RFR: 8258477: Pre-submit testing using GitHub Actions should merge changes from target branch

[Robin Westberg]

Normally when running GitHub Actions on a pull request, what is checked out is the merge of the pull request with the latest changes on the target branch. This ensure that what is tested is as close as possible to what will actually be the result of integrating said pull request. 

In our use of GitHub Actions, we don't run directly on pull requests but instead allow contributors to run them in their own personal forks. In that context, there is no notion of a target branch. However, we can infer the target project and branch by encoding this in the workflow file. This allows us to perform the same merge manually, and achieve the same behaviour. 

The change also adds an option to disable this automated merge when launching the workflow manually, which could be useful when investigating unexpected test failures.

Note that downstream projects picking up this change will have to adapt the workflow file to keep using these actions for pre-submit testing. This has been a common request from downstream projects, but could also be done in a separate change (or not at all).

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Commit messages:
 - Allow opting out of automated merge on manual runs
 - Initial implementation

Changes: https://git.openjdk.java.net/jdk/pull/1801/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=1801&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8258477
  Stats: 117 lines in 1 file changed: 116 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/1801.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/1801/head:pull/1801

PR: https://git.openjdk.java.net/jdk/pull/1801