When all files from some subdirectory were renamed to the root
directory, the directory rename heuristics would fail to detect that
as a rename/merge of the subdirectory to the root directory, which has
been corrected.

* en/merge-recursive-directory-rename-fixes:
  t604[236]: do not run setup in separate tests
  merge-recursive: fix merging a subdirectory into the root directory
  merge-recursive: clean up get_renamed_dir_portion()