Add feature to stop tracking files while keeping them in the index

Hello everyone,

Currently, a file can be either tracked or untracked. So, I propose we add a feature to stop tracking files while keeping them in the index.

Example scenario:
- A developer would like to add some configuration files with example
values, e.g. DB_PASSWORD=changeme. But in the future he also want to
update those files and to notify the others with conflicts.

Best regards,

Viet Nguyen