warn about old base revisions

[Archaeopteryx]

If a patch is submitted based on an old base revisions,, it inherits its issue and can be affected by new ones which depend on time, e.g. outdated certificates, test suites, documents (e.g. perfdocs).

4 weeks (1 development cycle) could be a threshold but the certificates which will be expiring are updated in the 3 weeks up to that date, hence 1 week might be better as threshold.

[marco-c]

We could also warn if the patch fails to apply to tip, which is a good indication that the patch might have conflicts other than just in applying.