The explanation for 'git commit --amend' talks about preparing a tree
object, which shouldn't be how user-facing documentation talks about
commit.
Reword it to say it works as usual, but replaces the current commit.
Signed-off-by: Carlos Martín Nieto <cmn@elego.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>