Add -tracked flag to ignore git-untracked files

[andrew]

Closes #57.

Long-lived worktrees accumulate stray files that aren't part of the project: a package.json from a one-off experiment, htmlcov/ from a coverage run, build artifacts that aren't gitignored. These currently confuse detection and report languages and package managers that have nothing to do with the actual project.

The new -tracked flag restricts scanning to files that git ls-files reports. It runs once at engine start, builds a set of tracked paths and their parent directories, and filters every file existence check and directory walk through it. Untracked subtrees are skipped entirely via SkipDir so there's no cost to having a large htmlcov/ or similar lying around.

Using -tracked outside a git repository is an error.

Before, with an untracked package-lock.json and htmlcov/*.js in this repo:

Language:        Go (also: JavaScript)

After, with -tracked:

Language:        Go