Fixes #5254: update cfclerk version to 2.12.0

There seems to have a change in the semantic of "path" in JGIT treewalks that no longer include a leading "/".

So, in place of "/category.xml" for the root category, we have to look for "category.xml" - nothing change for sub-categories, as we are comparing for the suffixe "/category.xml" in "..../category.xml". 

So, I added a special case for files under root in the FileTreeFilter, and also added a test for a descriptor trying to cheat with a name ending with "category.xml".