squash! rebase -i: reword executes pre-commit hook on interim commit

rebase -i: do not verify reworded patches using pre-commit

The to-do list command `reword` replays a commit like `pick` but lets
the user also edit the commit's log message. This happens in two
steps. Firstly, the named commit is cherry-picked. Secondly, the
commit created in the first step is amended using an unchanged index
to edit the log message. The pre-commit hook is meant to verify the
changes introduced by a commit (for instance, catching inserted
trailing white space). Since the committed tree is not changed
between the two steps and we do not verify rebased patches, do not
execute the pre-commit hook in the second step.

Specify the git-commit option `--no-pre-commit` to disable the
pre-commit hook when editing the log message. The commit-msg hook
will still be executed to verify the edited commit log message. As
before, if the hook finds the new log message ill-formatted, the
rebase will be interrupted with the unchanged commit replayed and the
new log message in `$GIT_DIR/COMMIT_EDITMSG`.

Add tests.