This has been broken since its introduction due to a typo.
`lib.cleanSourceWith` uses the field `origSrc` to store the initial path
prior to processing— which is presumably what the author intended.
With this change, `gitignoreSource (lib.sources.trace /path/to/repo)` no
longer incorrectly prints ignored paths.
Change-Id: I475cee9576dd9969f5fe5127d937bc7fb480eb22