rebase: drop '-i' from the reflog for interactive-based rebases

rebase: drop '-i' from the reflog for interactive-based rebases

A large variety of rebase types are supported by the interactive
machinery, not just the explicitly interactive ones.  These all share
the same code and write the same reflog messages, but the "-i" moniker
in those messages doesn't really have much meaning.  It also becomes
somewhat distracting once we switch the default from the am-backend to
the interactive one.  Just remove the "-i" from these messages.

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>