git-rebase -i: add command "drop" to remove a commit

git-rebase -i: add command "drop" to remove a commit

Instead of removing a line to remove the commit, you can use the
command "drop" (just like "pick" or "edit"). It has the same effect as
deleting the line (removing the commit) except that you keep a visual
trace of your actions, allowing a better control and reducing the
possibility of removing a commit by mistake.

Signed-off-by: Galan Rémi <remi.galan-alfonso@ensimag.grenoble-inp.fr>
Signed-off-by: Junio C Hamano <gitster@pobox.com>