Patterns would need to be read from:

• .gitignore file in the same directory as the path, or in any parent directory (the latter allows tools to put a .gitignore file with * in the directory they create to auto-exclude themselves)
• patterns read from $GIT_DIR/info/exclude (allows user-specific patterns for a specific repository)
• patterns read from the file specified by Git's configuration variable core.excludesFile (allows global user-specific patterns, I personally use this one)

Reference: https://git-scm.com/docs/gitignore

This might come at a performance cost but it definitely would be a useful feature.