rebase: decouple --exec from --interactive

rebase: decouple --exec from --interactive

In the later steps of preparing a patch series I do not want to
edit or reorder the patches any more, but just make sure the
test suite passes after each patch and also to fix breakage
right there if some of the steps fail.  I could run

    EDITOR=true git rebase -i <anchor> -x "make test"

but it would be simpler if it can be spelled like so:

    git rebase <anchor> -x "make test"

Signed-off-by: Stefan Beller <redacted>
Signed-off-by: Junio C Hamano <redacted>