Determines how pull requests are discovered:
Merging the pull request with the current target branch revision
Discover each pull request once with the discovered revision corresponding to the result of merging with the current revision of the target branch. Note that pushes to the target branch will result in new pull request builds.
The current pull request revision
Discover each pull request once with the discovered revision corresponding to the pull request head revision without merging.
Both the current pull request revision and the pull request merged with the current target branch revision
Discover each pull request twice. The first discovered revision corresponds to the result of merging with the current revision of the target branch in each scan. The second parallel discovered revision corresponds to the pull request head revision without merging.