commit/status: show the index-worktree diff with -v -v

commit/status: show the index-worktree diff with -v -v

git commit and git status in long format show the diff between HEAD
and the index when given -v. This allows previewing a commit to be made.

They also list tracked files with unstaged changes, but without a diff.

Introduce '-v -v' which shows the diff between the index and the
worktree in addition to the HEAD index diff. This allows a review of unstaged
changes which might be missing from the commit.

In the case of '-v -v', additonal header lines

Changes to be committed:

and

Changes not staged for commit:

are inserted before the diffs, which are equal to those in the status
part; the latter preceded by 50*"-" to make it stick out more.

Signed-off-by: Michael J Gruber <git@drmicha.warpmail.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>