rebase -i: Teach do_pick the option --edit

The todo list command `reword` replays a commit like `pick` but lets the
user also edit the commit's message. If one thinks of `pick` entries as
scheduled `cherry-pick` command lines, then `reword` becomes an alias
for the command line `cherry-pick --edit`. The porcelain
`rebase--interactive` defines a function `do_pick` for processing the
`pick` entries on todo lists. Teach `do_pick` to handle the option
`--edit` and reimplement `reword` in terms of `do_pick --edit`.

`do_pick` ought to act as a wrapper around `cherry-pick`. However, it
cannot just forward `--edit` to the `cherry-pick` command line it
assembles because the command is executed within a command substitution
for controlling the verbosity of `rebase--interactive`. Passing `--edit`
would either hang the terminal or clutter the substituted command output
with control sequences. Execute the old `reword` code instead when
`--edit` is specified.