git secrets --scan on repo where files have spaces in their names fails

[mtdowling]

This results in treating each space delimited segment of the filename as a separate file, causing file not found errors with grep.

When calling --scan with no arguments, we scan all files returned by git ls-files. We might need to re-think this or to escape these filenames when passing them to grep. We could also look into using git grep in this case.