commit: add amend suboption to --fixup to create amend! commit

commit: add amend suboption to --fixup to create amend! commit

`git commit --fixup=amend:<commit>` will create an "amend!" commit.
The resulting commit message subject will be "amend! ..." where
"..." is the subject line of <commit> and the initial message
body will be <commit>'s message.

The "amend!" commit when rebased with --autosquash will fixup the
contents and replace the commit message of <commit> with the
"amend!" commit's message body.

In order to prevent rebase from creating commits with an empty
message we refuse to create an "amend!" commit if commit message
body is empty.

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