squash! rebase -i: Commit only once when rewriting picks

git-commit lets the user edit the commit log message by default. We do
not want that default behaviour for the rewriting git-commit command
line. Pass `--no-edit` when rewriting a picked commit. An explicit
`--edit` passed to `do_pick` (for instance, when reword is executed)
enables the editor launch again.

TODO: Document the changed behaviour for root commit picks.