Git trees are now always lazily sorted

Git trees are now always lazily sorted

Removed `git_tree_add_entry_unsorted`. Now the `git_tree_add_entry`
method doesn't sort the entries array by default; entries are only
sorted lazily when required. This is done automatically by the library
(the `git_tree_sort_entries` call has been removed).

This should improve performance. No point on sorting entries all the time, anyway.

Signed-off-by: Vicent Marti <tanoku@gmail.com>