algos to sort the entries:readme-edits

I found the problem: git_tree.entries uses 2 different algos to sort the entries:

entry_search_cmp and entry_sort_cmp
entry_search_cmp makes an alphabetic sort, but entry_sort_cmp is more sophisticated (directories and files are not sorted the same way).

So i see 3 solutions:
1- We need to sort the list alphabetically before make the search (problem: git_vector_bsearch2 makes a sort)
2- We split entry_search_cmp into 2 functions: entry_search_file_cmp and entry_search_dir_cmp, we use gitfo_cmp_path and we search for a file and after for a directory.
3- Maybe we can always (everywhere) use an alphabetic sort???
4- An other better solution, ...

I made this patch:

https://github.com/versmisse/libgit2/commit/8ee19a25ab4b54ff583bb46093610db0f53d2ab5

to test the solution and it works fine with git_tree_entry_byname. But it is not very clean (i change a private member (_cmp)) and incomplete (we must find a solution for all bsearch2 in the code).

Originally posted by @versmisse in https://github.com/libgit2/libgit2/issues/127#issuecomment-1038775

Start with git_tree.entries, entry_search_cmp, entry_sort_cmp, and the git_vector_bsearch2 calls described in the issue. Compare the linked patch and determine a consistent search and sorting strategy for git_tree_entry_byname and the other bsearch2 uses; done means the entry lookup behavior is correct without changing a private member as a workaround.