Update `sources` URL refs from Renovate Bot

[OmarTawfik]

Follow up from https://github.com/cashapp/hermit/issues/342

Users can use a specific git reference to fix their package sources in hermit.hcl sources property. Example:

# hermit.hcl
sources = [
  "https://github.com/cashapp/hermit-packages.git#ref"
]

This means it will quickly get out of date, and it needs updating regularly.

I suggest adding support for this in the Renovate bot, by updating the ref suffix part of the URL regularly: https://docs.renovatebot.com/modules/manager/hermit/

Note: for most use cases, this means fetching the latest commit from the default branch of the repository they are using (origin/HEAD), but for completeness, it can support specifying a custom remote reference (like refs/HEAD/branch-name) for users to put into their renovate.json config, for repositories that use a separate release branch.