git-blame.el: separate git-blame-mode to ease maintenance

git-blame.el: separate git-blame-mode to ease maintenance

git-blame-mode has been splitted into git-blame-mode-on and
git-blame-mode-off; it now conditionnaly calls one of them depending
of how we call it. Code is now easier to maintain and to understand.

Fixed `git-reblame' function: interactive form was at the wrong
place.

String displayed on the mode line is now configurable through
`git-blame-mode-line-string` (default to " blame").

Signed-off-by: Xavier Maillard <zedek@gnu.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>