Declare ff from archive/debian/4.11.1_pre+1.733450b39b-1_exp1

This converts the branch to a form which will be ff from
dgit/dgit/experimental, when we dgit push.

I have checked that this is sane with this rune:

  git-diff archive/debian/4.11.1_pre+1.733450b39b-1_exp1 -- :/ :!/debian/patches

The only output is .gitignore, which the previous source construction
approach entirely left out of the .dsc.

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>