status: better advices when splitting a commit (during rebase -i)

status: better advices when splitting a commit (during rebase -i)

Add new informative help messages at the output of 'git status' when
the user is splitting a commit. The code figures this state by
comparing the contents of the following files in the .git/ directory:
	  - HEAD
	  - ORIG_HEAD
	  - rebase-merge/amend
	  - rebase-merge/orig-head

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>