git-commit: show dirtiness including index.

git-commit: show dirtiness including index.

Earlier, when we switched a branch we used diff-files to show
paths that are dirty in the working tree.  But we allow switching
branches with updated index ("read-tree -m -u $old $new" works that
way), and only showing paths that have differences in the working
tree but not paths that are different in index was confusing.

This shows both as modified from the top commit of the branch we
just have switched to.

Signed-off-by: Junio C Hamano <junkio@cox.net>