pc should have option to ignore errors from files not known to git

Sometimes you don't care that the file isn't known to git. This is frequently the case with a plugin that may not be checking.

We should add a new option that indicates that `pc` should _not_ return an exit status indicating failure if the only problem is that the file isn't known to git.

Instead it should return the same thing as a request for a file known to git with no comments in it.

maybe `-q` && `--quiet` ? I'm not sure if "quiet" is a good thing because its a word whose meaning might expand over time. `-g` & `--gitless` ? dunno, that seems worse. I'm open to suggestions.

Begin at the pc command-line entry point and trace how it handles files unknown to git. Compare that path with a known Git file that has no comments; done means an option suppresses only the unknown-file failure and preserves the normal no-comment result.

Written by the indexing model from the issue text.