detect when no new commit/Hackage changes to avoid unnecessary runs

[juhp]

Currently we just loop nightly and it keeps running build.sh (after 49min sleep currently) even if there are no changes.
This is a waste of resources, time and energy: we should skip when there are no changes since the last build.

However it is not trivial, we need to detect:

• if Hackage was updated for any packages in Stackage?

or

• if there were any new stackage commits?

No sure if there are any others important changing conditions.

Though with longer sleep times this might be less of an issue.