builtin rebase: support `--exec`

builtin rebase: support `--exec`

This commit adds support for the `--exec` option which takes a shell
command-line as argument. This argument will be appended as an `exec
<cmd>` command after each line in the todo list that creates a commit in
the final history.  commands.

Note: while the shell script version of `git rebase` assigned the empty
string to `cmd` by default, we *unset* it here because the code looks
nicer and it does not change the behavior.

The `--exec` option requires `--interactive` machinery.

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>