git-commit: document --amend

git-commit: document --amend

The "--amend" option is used to amend the tip of the current branch. This
documentation text was copied straight from the commit that implemented it.

Some minor format tweaks for asciidoc were taken from work by Francis Daly
in commit b0d08a5.. It looks good now also in the html page.

[jc: amended further to follow the recommendation by Francis in
commit 3070b60].

Signed-off-by: Marco Roeland <marco.roeland@xs4all.nl>
Signed-off-by: Junio C Hamano <junkio@cox.net>