This makes tree iterators directly support case insensitivity by
using a secondary index that can be sorted by icase.  Also, this
fixes the ambiguity check in the git_status_file API to also be
case insensitive.  Lastly, this adds new test cases for case
insensitive range boundary checking for all types of iterators.

With this change, it should be possible to deprecate the spool
and sort iterator, but I haven't done that yet.