Add payload "_r" versions of bsearch and tsort

git__bsearch and git__tsort did not pass a payload through to the
comparison function.  This makes it impossible to implement sorted
lists where the sort order depends on external data (e.g. building
a secondary sort order for the entries in a tree).  This commit
adds git__bsearch_r and git__tsort_r versions that pass a third
parameter to the cmp function of a user payload.
Add git_path_icmp to case-insensitive path cmp

This adds git_path_icmp to complement git_path_cmp.
Add git_tree_entry_cmp and git_tree_entry_icmp

This adds a new external API git_tree_entry_cmp and a new internal
API git_tree_entry_icmp for sorting tree entries.  The case
insensitive one is internal only because general users should
never be seeing case-insensitively sorted trees.
Support case insensitive tree iterators and status

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.