gitignore: Do *not* ignore the debian/ folder

Forward port of the relevant part of !122 (merged).

The upstream .gitignore fully excludes /debian/ because it considers it the work area for make deb-pkg.

In Apertis we do not care about make deb-pkg but we absolutely care about the packaging metadata derived from Debian so having it ignored by default is extremely error prone.

In fact, this has been the cause of breakage in a few instances, see the previous commit.

While editing files outside of debian/ is usually forbidden by the Debian packaging tools, .gitignore is part of the default dpkg-source -i exceptions, see https://bugs.debian.org/409566.