Using Python 3.5+ os.scandir() optimized variant.
Starting.
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*Collecting tree differences
Skipping system cruft (metadata) files in both FROM and TO
comparing [C:/MY-STUFF/Code/mergeall/test/test1] [C:/MY-STUFF/Code/mergeall/test/test2]
excluding C:/MY-STUFF/Code/mergeall/test/test2\__bkp__
comparing [C:/MY-STUFF/Code/mergeall/test/test1\d3] [C:/MY-STUFF/Code/mergeall/test/test2\d3]
Phase runtime: 0.0036746145662131923
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*Reporting tree differences
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SAMEFILE DIFFERENCES: (name, dirfrom, dirto, why)
**These items will be replaced in dirto by automatic resolution**

[('.htaccess',
  'C:/MY-STUFF/Code/mergeall/test/test1\\d3',
  'C:/MY-STUFF/Code/mergeall/test/test2\\d3',
  'modtime'),
 ('f3.txt',
  'C:/MY-STUFF/Code/mergeall/test/test1\\d3',
  'C:/MY-STUFF/Code/mergeall/test/test2\\d3',
  'modtime'),
 ('f3.txt',
  'C:/MY-STUFF/Code/mergeall/test/test1',
  'C:/MY-STUFF/Code/mergeall/test/test2',
  'modtime'),
 ('f6.txt',
  'C:/MY-STUFF/Code/mergeall/test/test1',
  'C:/MY-STUFF/Code/mergeall/test/test2',
  'filesize')]
-------------------------------------------------------------------------------
UNIQUE ITEMS IN DIRTO: (names, dirfrom, dirto)
**These items will be deleted from dirto by automatic resolution**

[(['d2', 'f2.txt'],
  'C:/MY-STUFF/Code/mergeall/test/test1',
  'C:/MY-STUFF/Code/mergeall/test/test2'),
 (['f5.txt'],
  'C:/MY-STUFF/Code/mergeall/test/test1\\d3',
  'C:/MY-STUFF/Code/mergeall/test/test2\\d3')]
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UNIQUE ITEMS IN DIRFROM: (names, dirfrom, dirto)
**These items will be copied over to dirto by automatic resolution**

[(['d1', 'f1.txt'],
  'C:/MY-STUFF/Code/mergeall/test/test1',
  'C:/MY-STUFF/Code/mergeall/test/test2'),
 (['f4.txt'],
  'C:/MY-STUFF/Code/mergeall/test/test1\\d3',
  'C:/MY-STUFF/Code/mergeall/test/test2\\d3')]
-------------------------------------------------------------------------------
MIXED MODE NAMES: (name, dirfrom, dirto)
**These items will be replaced in dirto by automatic resolution**

[('m1',
  'C:/MY-STUFF/Code/mergeall/test/test1',
  'C:/MY-STUFF/Code/mergeall/test/test2'),
 ('m2',
  'C:/MY-STUFF/Code/mergeall/test/test1',
  'C:/MY-STUFF/Code/mergeall/test/test2')]
------------------------------------------------------------------------------- 
*Summary
Compared    => files: 6, folders: 2
Differences => samefile: 4, uniqueto: 3, uniquefrom: 3, mixedmode: 2
Changed:
files   => created: 0, deleted: 0, replaced: 0
folders => created: 0, deleted: 0, replaced: 0
-------------------------------------------------------------------------------
Finished.