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/odb.c b/src/odb.c
index b09931660..caa4e1bef 100644
--- a/src/odb.c
+++ b/src/odb.c
@@ -223,8 +223,8 @@ static int init_fake_wstream(git_odb_stream **stream_p, git_odb_backend *backend
 
 static int backend_sort_cmp(const void *a, const void *b)
 {
-	const backend_internal *backend_a = *(const backend_internal **)(a);
-	const backend_internal *backend_b = *(const backend_internal **)(b);
+	const backend_internal *backend_a = (const backend_internal *)(a);
+	const backend_internal *backend_b = (const backend_internal *)(b);
 
 	if (backend_a->is_alternate == backend_b->is_alternate)
 		return (backend_b->priority - backend_a->priority);