sequencer: allow the commit-msg hooks to run during a `reword`

sequencer: allow the commit-msg hooks to run during a `reword`

The `reword` command used to call `git commit` in a manner that asks for
the prepare-commit-msg and commit-msg hooks to do their thing.

Converting that part of the interactive rebase to C code introduced the
regression where those hooks were no longer run.

Let's fix this.

Note: the flag is called `VERIFY_MSG` instead of the more intuitive
`RUN_COMMIT_MSG_HOOKS` to indicate that the flag suppresses the
`--no-verify` flag (which may do other things in the future in addition
to suppressing the commit message hooks, too).

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