If you every copy or rename a git pathname contains a space, then the
git-fast-import transfer format will not be able to deal with that case.
This is a limitation in the design of git-fast-import.

But as long as you do not copy or rename files with spaces in their
pathnames, git-fast-import and also Fossil should now be able to deal with,
at least as of check-in [4ee4aa5a30733a5].