rebase -i: add fixup [-C | -c] command

rebase -i: add fixup [-C | -c] command

Add options to `fixup` command to fixup both the commit contents and
message. `fixup -C` command is used to replace the original commit
message and `fixup -c`, additionally allows to edit the commit message.

Original-patch-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Mentored-by: Christian Couder <chriscool@tuxfamily.org>
Mentored-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Signed-off-by: Charvi Mendiratta <charvi077@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>