Fix for KFileItemModel::expansionLevelsCompare

Fix for KFileItemModel::expansionLevelsCompare

Before this commit, expanding and collapsing folders in the details
view would lead to strange results in a folder with the items "a",
"a/a/", "a/a/1", "a/a-1/", and "a/a-1/1".  The problem was that the
comparison between "a/a/1" and "a/a-1/1" went wrong: the first
character in which the paths differ is a "/" in one of the items, such
that the code that reduces this 'index' in
KFileItemModel::expansionLevelsCompare in order to find the 'common
path' did nothing because it checked that only *one* of the two items
does not have an "/" at the position 'index'.