test suite: git-debrebase: Do some tests without dgit installed

test suite: git-debrebase: Do some tests without dgit installed

We provide a new psuedo-dependency NO-DGIT which throws away the usual
default dependency list.

We abolish NO-DEFAULT which could not be useful because the tests
pretty much all depend on setup/gnupg which (currently, as a
workaround for the gnupg2 races) requires chiark-utils-bin for
with-lock-ex.

There are two tests which don't actually need any interaction with the
archive.  Mark them accordingly.

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