Append commit hash to `--version` output when building from HEAD

With the recent macOS Tahoe upgrade, there is an increase in issues as we uncover problems we need to work around (e.g., #2638) Users can report “I'm running 7.1.15” — but that doesn’t distinguish between official releases and custom HEAD builds. Appending the commit hash makes it immediately clear exactly what code a user is running, especially as incremental patches are added over the coming weeks. Since the adding of the commit is optional, it doesn't need to be included for release builds. Future improvements could include appending a -dirty suffix if the working tree has uncommitted changes, but that’s left out here for simplicity. I just built from HEAD, and get: ```sh $ ./bin/yabai --version yabai-v7.1.15-46ef675 ```