Provide flake's current commit message in self.sourceInfo

Is your feature request related to a problem?

It would be great if one could access the current commit message of the flake. The self.sourceInfo.*Rev are great, but for things like the boot menu or other version identifiers consumed by humans, a commit message would be much more readable.

Proposed solution

New self.sourceInfo string attributes:

• commitTitle: first line of the current flake commit
• commitMsg: full commit message, possibly multiple lines

If necessary, the same dirty* variants for dirty flake trees.

Alternative solutions

There are partial workarounds to this, that are rather impure and require external setup:

• using a git hook to write the commit message to file, then read it with nix: https://discourse.nixos.org/t/cannot-access-commit-message-from-inside-build-sandbox/59138/9
• set system.nixos.label or the NIXOS_LABEL envvar: https://www.reddit.com/r/NixOS/comments/xi7gke/nixos_boot_menu_entries_based_on_current_git/

Additional context

Checklist

• checked latest Nix manual (source)
• checked open feature issues and pull requests for possible duplicates

Add 👍 to issues you find important.

Start with the latest Nix manual and the existing self.sourceInfo.*Rev attributes to understand how flake source metadata is exposed. Define the behavior of commitTitle and commitMsg, including whether dirty variants are required, then verify that the attributes provide the current commit information for human-readable identifiers.