Previously we assumed that all input and ouput paths for broccoli trees are immutable, that turned out to be
incorrect.
By adding a tree stabilizer plugin in front of each diffing plugin, we ensure that the input trees
are stable. The stabilization is done via symlinks which is super cheap on platforms that support
symlinks. On Windows we currently copy the whole input directory, which is far from ideal. We should
investagate if using move operation on Windows is ok in the future to improve performance.
Closes#2051
Igor Minar