rebase -i: teach "--exec <cmd>"

rebase -i: teach "--exec <cmd>"

During an interactive rebase session, it is sometimes desirable to
run tests on each commit in the resulting history.  This can be done
by adding "exec <test command>" when editing the insn sheet, but the
command used for testing is often the same for all resulting commits.

By passing "--exec <cmd>" from the command line, automatically add
these "exec" lines after each commit in the final history.  To work
well with the --autosquash option, these are added at the end of
each run of "fixup" and "squash".

Helped-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Lucien Kong <Lucien.Kong@ensimag.imag.fr>
Signed-off-by: Valentin Duperray <Valentin.Duperray@ensimag.imag.fr>
Signed-off-by: Franck Jonas <Franck.Jonas@ensimag.imag.fr>
Signed-off-by: Thomas Nguy <Thomas.Nguy@ensimag.imag.fr>
Signed-off-by: Huynh Khoi Nguyen Nguyen <Huynh-Khoi-Nguyen.Nguyen@ensimag.imag.fr>
Signed-off-by: Matthieu Moy <Matthieu.Moy@grenoble-inp.fr>
Signed-off-by: Junio C Hamano <gitster@pobox.com>