Provide a helper function that import scripts can use when the
third-party project defines custom Git attributes in its top-level
`.gitattributes` file. These must be commented out because Git
does not support custom attributes in a subdirectory where we
where the third-party project is about to be merged.
This should be done so that any attributes for ignoring certain files
when exporting can be appended to the file during the extraction step,
but ignored in the actual import. Necessary for importing the gitsetup
repository.
The `git ls-files | xargs rm` removes only files; directories which are
empty are left laying around. This later chokes the `mv` which puts the
"reduced" directory into place. Remove the empty directories as well.
