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 <redacted>
Signed-off-by: Valentin Duperray <redacted>
Signed-off-by: Franck Jonas <redacted>
Signed-off-by: Thomas Nguy <redacted>
Signed-off-by: Huynh Khoi Nguyen Nguyen <redacted>
Signed-off-by: Matthieu Moy <redacted>
Signed-off-by: Junio C Hamano <redacted>