builtin rebase: use no-op editor when interactive is "implied"

builtin rebase: use no-op editor when interactive is "implied"

Some options are only handled by the git-rebase--interactive backend,
even if run non-interactively. For this awkward situation (run
non-interactively, but use the interactive backend), the shell scripted
version of `git rebase` introduced the concept of an "implied
interactive rebase". All it does is to replace the editor by a dummy one
(`:` is the Unix command that takes arbitrary command-line parameters,
ignores them and simply exits with success).

Signed-off-by: Pratik Karki <predatoramigo@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>