Respect --force-exclude for files passed via stdin

[charliermarsh]

Resolves #1339.

[charliermarsh]

It turns out that it's very, very hard to use the ignore crate to answer the question, "Is a given path ignored?", because it doesn't seem to expose the underlying matchers, only the directory walkers. This creates a few problems. I'll just leave this comment here for posterity as I roll back some of the code:

// Step 2: Is the file ignored via a gitignore?
// Note that we can't enforce this behavior for non-existent files, which _could_ come up when
// you pass a file via `--stdin-filename`, and _could_ be considered incorrect. For example,
// if `subdir` is listed in the `gitignore`, and you pass `subdir/non_existent_file.py`, then
// right now, we _wouldn't_ mark that theoretical file as ignored. If the `ignore` crate had
// a public matcher API, we could support that, but right now, we _have_ to look at the
// filesystem to reverse-engineer the gitignore match.

// TODO(charlie): This isn't right either, because the _parent_ could be the thing that's
// ignored via the `.gitignore`.

[jhossbach]

Seems like I was too slow. Also closes #1343

[charliermarsh]

Sadly this doesn't respect gitignore rules in those cases (I tried to get this to work for a while, but I don't think it's reliable without changes to the ignore crate), but otherwise seems to work.