Support multiple gitignores

[davidmfoley]

When there are gitignores in both /project-root/.gitignore and /project-root/subfolder/.gitignore only the first .gitignore found, walking the tree up from the base (pwd) is used to determine excluded files.

This results in two bugs:

• When running from /project-root/, only the higher-level .gitignore is parsed, so the files in /project-root/subfolder are not properly filtered
• Conversely, when running from /project-root/subfolder, the higher-level ignores in /project-root are not read

[davidmfoley]

Also global gitignores are not supported. If a git project is detected, we may wish to just use git ls-files and leave this to git.