Source updates should record the change date

[bb010g]

The convention for unstable packages is to use the last modification date as version. Niv should assist in setting this by recording it when updating in a property like date.

For VCS sources, this should be the date of the latest change (e.g. Git's commit date). For file sources, this can default to the current date. Datestamps should be normalized to UTC.

[nmattia]

Nice! Currently niv uses tarball releases from github, I’m not sure how we could grab the last commit date from there. It should probably just use nix-prefetch-git instead, which would also work around #62 though would hit performance quite heavily for big repos.

[bb010g]

It's not a drop-in solution for Niv, but here's a script I've hacked together to emulate this with GitHub's v4 API: update-commit-dates.sh, commit-dates-query.jq, commit-dates-filter.jq

It runs pleasantly quickly. (This'll silently break if your Niv sources have underscores in them, but an actual solution could just index and avoid some extra bookeeeping by not being constructed out of curl and two separate jq invocations.)

[Nadrieril]

Storing the current date would already be very useful. Maybe that's simpler as a first step ?

[Nadrieril]

I mean just storing the date of the last time an entry got niv updated would be already really helpful. Getting a more precise date from nix-prefetch-git could be done later on.
I wanted to try making a PR but I honestly can't follow the update code :(.