Changed approach to files limited project. Now we keep into the
top-level dir a list of subdirectories to be shown. When in file
limited mode we will not scan subdirectories unless they are in
the list of shown subdirectories.
With the new mechanism the function get_subdirectory_files always
recurse into subdirectories by default but is able to figure out
to stop recursing into subdirectories for files limited project.
The new mechanism is more robust of the previous one. Now the
rescan of subdirectories is compatible with files limited project.
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