git-rebase, sequencer: extend --quiet option for the interactive machinery

git-rebase, sequencer: extend --quiet option for the interactive machinery

While 'quiet' and 'interactive' may sound like antonyms, the interactive
machinery actually has logic that implements several
interactive_rebase=implied cases (--exec, --keep-empty, --rebase-merges)
which won't pop up an editor.  The rewrite of interactive rebase in C
added a quiet option, though it only turns stats off.  Since we want to
make the interactive machinery also take over for git-rebase--merge, it
should fully implement the --quiet option.

git-rebase--interactive was already somewhat quieter than
git-rebase--merge and git-rebase--am, possibly because cherry-pick has
just traditionally been quieter.  As such, we only drop a few
informational messages -- "Rebasing (n/m)" and "Successfully rebased..."

Also, for simplicity, remove the differences in how quiet and verbose
options were recorded.  Having one be signalled by the presence of a
"verbose" file in the state_dir, while the other was signalled by the
contents of a "quiet" file was just weirdly inconsistent.  (This
inconsistency pre-dated the rewrite into C.)  Make them consistent by
having them both key off the presence of the file.

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