Git-Dch: ignore

Conflicts:
	debian/changelog

This commit also adjusts and extends the test suite accordingly.

Contributors doc: make it clear that overlay APT suites are *not* added when building a tagged release.

Test suite: take into account that experimental isn't a base branch anymore, but feature/jessie now is one.

It does *not* contain the full list of APT suites we'll be adding
dynamically at build time, so let's make it clearer that it "merely"
contains a list of overlays (that will be used in addition to e.g.
the base branch's APT suite).

One branch has to be the first to ship this file.

Embed the base branch name and its top commit into the generated filename, when building from a topic branch in Jenkins.

This will improve debugging, at least in Jenkins.

... unless we're building from a tag, or from the base branch itself.

I'll need that function soon.

And update the ticket reference to what's remaining to automate.

This reverts commit 986b9d46.

Actually, we only automatically test it in English, so the manual test
is still required.