revert/cherry-pick: allow starting from dirty work tree.

revert/cherry-pick: allow starting from dirty work tree.

There is no reason to forbid a dirty work tree when reverting or
cherry-picking a change, as long as the index is clean.

The scripted version used to allow it:

    case "$no_commit" in
    t)
    	# We do not intend to commit immediately.  We just want to
    	# merge the differences in.
    	head=$(git-write-tree) ||
    		die "Your index file is unmerged."
    	;;
    *)
    	head=$(git-rev-parse --verify HEAD) ||
    		die "You do not have a valid HEAD"
    	files=$(git-diff-index --cached --name-only $head) || exit
    	if [ "$files" ]; then
    		die "Dirty index: cannot $me (dirty: $files)"
    	fi
    	;;
    esac

but C rewrite tightened the check, probably by mistake.

Signed-off-by: Junio C Hamano <gitster@pobox.com>