Check for last built revision in the current branch

Currently the isRevExcluded method in GitSCM checks for changes by using the last built revision and the last commit. This breaks in our setup - we have multiple branches with multiple projects that are all seperated via included regions.

If a project has been built in branch A and another commit happens in branch B of an unrelated project, the project starts another build for a revision that has already been built previously for branch A.

This patch changes isRevExcluded to no longer use the last built revision but the last built revision for the current branch. There is also a test case included that should demonstrate the behaviour / fail with the old code.

The patch also adds some more logging we used to track down the problem, I can remove that if requested.