builtin rebase: support --quiet

builtin rebase: support --quiet

This commit introduces a rebase option `--quiet`. While `--quiet` is
commonly perceived as opposite to `--verbose`, this is not the case for
the rebase command: both `--quiet` and `--verbose` default to `false` if
neither `--quiet` nor `--verbose` is present.

Despite the default being `false` for both verbose and quiet mode,
passing the `--quiet` option will turn off verbose mode, and `--verbose`
will turn off quiet mode.

This patch introduces the `flags` bit field, with `REBASE_NO_QUIET`
as first user (with many more to come).

We do *not* use `REBASE_QUIET` here for an important reason: To keep the
implementation simple, this commit introduces `--no-quiet` instead of
`--quiet`, so that a single `OPT_NEGBIT()` can turn on quiet mode and
turn off verbose and diffstat mode at the same time. Likewise, the
companion commit which will introduce support for `--verbose` will have
a single `OPT_BIT()` that turns off quiet mode and turns on verbose and
diffstat mode at the same time.

Signed-off-by: Pratik Karki <predatoramigo@gmail.com>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>