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$ git remote add -f Bproject /path/to/B <1>
-$ git merge -s ours --no-commit Bproject/master <2>
+$ git merge -s ours --no-commit --allow-unrelated-histories Bproject/master <2>
$ git read-tree --prefix=dir-B/ -u Bproject/master <3>
$ git commit -m "Merge B project as our subdirectory" <4>