Git/Hg are just overkill for a single text file.

Overkill by which metric? You don't need to install any server to use them, the only real downside is that you should keep the file in a separate directory.

But if you use git or hg for other projects anway, they are installed, and typing 'git init' isn't really all that much extra work, and the few additional files won't fill up your hard disc anytime soon.