auto

Otherwise, for example we can hit this scenario:

1. build1: during a built from the stable branch, the website is built with
   SOURCE_DATE_EPOCH derived from the last changelog entry on that
   branch (date1); the result of that build is cached.

2. build2: during another build from the devel branch, on a different
   isobuilder, the website is built with SOURCE_DATE_EPOCH derived from the
   last changelog entry on that branch (date2); the result of that build is
   cached; it differs from the version cached during build1, due to a different
   SOURCE_DATE_EPOCH, because the last changelog entry on the devel branch
   has a different date than the last changelog entry on the stable branch.

3. When Jenkins tries to reproduce build2, on the same isobuilder as build1, our
   build system reuses the website built with date1, which does not match the
   website built with date2 during build 2 ⇒ different images, reproducibility
   check fails.

So let's take into account debian/changelog as part of the input
on which we key our built website cache.