Provide calculated branch to SLOOP

[georgii-borovinskikh-sonarsource]

Sloop has 2 methods: didChangeMatchedSonarProjectBranch that requests the client to calculate the branch and matchSonarProjectBranch that notifies the client about changing of the branch. Currently they always happen subsequently, but in the future the core will serve as the primary source of truth for the branch name, including possibly calculating the branch by itself. More here:https://sonarsource.atlassian.net/browse/SLCORE-570

Currently we're fine with just returning the master branch for the first method, but in the future it might be beneficial to integrate this properly.