We now store only one sorting callback that does entry comparison. This
is used when sorting the entries using a quicksort, and when looking for
a specific entry with the new search methods.

The following search methods now exist:

	git_vector_search(vector, entry)
	git_vector_search2(vector, custom_search_callback, key)

	git_vector_bsearch(vector, entry)
	git_vector_bsearch2(vector, custom_search_callback, key)

The sorting state of the vector is now stored internally.

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