pseudo-merge: implement support for finding existing merges

This patch implements support for reusing existing pseudo-merge commits
when writing bitmaps when there is an existing pseudo-merge bitmap which
has exactly the same set of parents as one that we are about to write.

Note that unstable pseudo-merges are likely to change between
consecutive repacks, and so are generally poor candidates for reuse.
However, stable pseudo-merges (see the configuration option
'bitmapPseudoMerge.<name>.stableThreshold') are by definition unlikely
to change between runs (as they represent long-running branches).

Because there is no index from a *set* of pseudo-merge parents to a
matching pseudo-merge bitmap, we have to construct the bitmap
corresponding to the set of parents for each pending pseudo-merge commit
and see if a matching bitmap exists.

This is technically quadratic in the number of pseudo-merges, but is OK
in practice for a couple of reasons:

  - non-matching pseudo-merge bitmaps are rejected quickly as soon as
    they differ in a single bit

  - already-matched pseudo-merge bitmaps are discarded from subsequent
    rounds of search

  - the number of pseudo-merges is generally small, even for large
    repositories

In order to do this, implement (a) a function that finds a matching
pseudo-merge given some uncompressed bitset describing its parents, (b)
a function that computes the bitset of parents for a given pseudo-merge
commit, and (c) call that function before computing the set of reachable
objects for some pending pseudo-merge.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

pack-bitmap.c

@@ -1316,6 +1316,37 @@ cleanup:
 	return cb.base;
 }
 
+struct ewah_bitmap *pseudo_merge_bitmap_for_commit(struct bitmap_index *bitmap_git,
+						   struct commit *commit)
+{
+	struct commit_list *p;
+	struct bitmap *parents;
+	struct pseudo_merge *match = NULL;
+
+	if (!bitmap_git->pseudo_merges.nr)
+		return NULL;
+
+	parents = bitmap_new();
+
+	for (p = commit->parents; p; p = p->next) {
+		int pos = bitmap_position(bitmap_git, &p->item->object.oid);
+		if (pos < 0 || pos >= bitmap_num_objects(bitmap_git))
+			goto done;
+
+		bitmap_set(parents, pos);
+	}
+
+	match = pseudo_merge_for_parents(&bitmap_git->pseudo_merges,
+						parents);
+
+done:
+	bitmap_free(parents);
+	if (match)
+		return pseudo_merge_bitmap(&bitmap_git->pseudo_merges, match);
+
+	return NULL;
+}
+
 static void unsatisfy_all_pseudo_merges(struct bitmap_index *bitmap_git)
 {
 	uint32_t i;
@@ -2809,6 +2840,7 @@ void free_bitmap_index(struct bitmap_index *b)
 		 */
 		close_midx_revindex(b->midx);
 	}
+	free_pseudo_merge_map(&b->pseudo_merges);
 	free(b);
 }