vector: Timsort all of the things

Drop the GLibc implementation of Merge Sort and replace it with Timsort.

The algorithm has been tuned to work on arrays of pointers (void **),
so there's no longer a need to abstract the byte-width of each element
in the array.

All the comparison callbacks now take pointers-to-elements, not
pointers-to-pointers, so there's now one less level of dereferencing.

E.g.

	 int index_cmp(const void *a, const void *b)
	 {
	-	const git_index_entry *entry_a = *(const git_index_entry **)(a);
	+	const git_index_entry *entry_a = (const git_index_entry *)(a);

The result is up to a 40% speed-up when sorting vectors. Memory usage
remains lineal.

A new `bsearch` implementation has been added, whose callback also
supplies pointer-to-elements, to uniform the Vector API again.

1 files changed, 2 insertions, 2 deletions

diff --git a/src/refs.c b/src/refs.c
index 84224e88e..5e0fe09b2 100644
--- a/src/refs.c
+++ b/src/refs.c
@@ -825,8 +825,8 @@ static int packed_remove_loose(git_repository *repo, git_vector *packing_list)
 
 static int packed_sort(const void *a, const void *b)
 {
-	const git_reference *ref_a = *(const git_reference **)a;
-	const git_reference *ref_b = *(const git_reference **)b;
+	const git_reference *ref_a = (const git_reference *)a;
+	const git_reference *ref_b = (const git_reference *)b;
 
 	return strcmp(ref_a->name, ref_b->name);
 }