bitmap: move `get commit positions` code to `bitmap_writer_finish`

The `write_selected_commits_v1` function takes care of writing commit
positions along with their corresponding bitmaps in the disk. It is
OK because this `search commit position of a given commit` algorithm
is needed only once here. But in later changes of the `lookup table
extension series`, we need same commit positions which means we have
to run the above mentioned algorithm one more time.

Move the `search commit position of a given commit` algorithm to
`bitmap_writer_finish()` and use the `commit_positions` array
to get commit positions of their corresponding bitmaps.

Signed-off-by: Abhradeep Chakraborty <chakrabortyabhradeep79@gmail.com>
Reviewed-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

pack-bitmap-write.c

@@ -650,20 +650,15 @@ static const struct object_id *oid_access(size_t pos, const void *table)
 
 static void write_selected_commits_v1(struct hashfile *f,
 				      struct pack_idx_entry **index,
-				      uint32_t index_nr)
+				      uint32_t index_nr,
+				      uint32_t *commit_positions)
 {
 	int i;
 
 	for (i = 0; i < writer.selected_nr; ++i) {
 		struct bitmapped_commit *stored = &writer.selected[i];
 
-		int commit_pos =
-			oid_pos(&stored->commit->object.oid, index, index_nr, oid_access);
-
-		if (commit_pos < 0)
-			BUG("trying to write commit not in index");
-
-		hashwrite_be32(f, commit_pos);
+		hashwrite_be32(f, commit_positions[i]);
 		hashwrite_u8(f, stored->xor_offset);
 		hashwrite_u8(f, stored->flags);
 
@@ -697,6 +692,8 @@ void bitmap_writer_finish(struct pack_idx_entry **index,
 	static uint16_t flags = BITMAP_OPT_FULL_DAG;
 	struct strbuf tmp_file = STRBUF_INIT;
 	struct hashfile *f;
+	uint32_t *commit_positions = NULL;
+	uint32_t i;
 
 	struct bitmap_disk_header header;
 
@@ -715,7 +712,20 @@ void bitmap_writer_finish(struct pack_idx_entry **index,
 	dump_bitmap(f, writer.trees);
 	dump_bitmap(f, writer.blobs);
 	dump_bitmap(f, writer.tags);
-	write_selected_commits_v1(f, index, index_nr);
+
+	ALLOC_ARRAY(commit_positions, writer.selected_nr);
+
+	for (i = 0; i < writer.selected_nr; i++) {
+		struct bitmapped_commit *stored = &writer.selected[i];
+		int commit_pos = oid_pos(&stored->commit->object.oid, index, index_nr, oid_access);
+
+		if (commit_pos < 0)
+			BUG(_("trying to write commit not in index"));
+
+		commit_positions[i] = commit_pos;
+	}
+
+	write_selected_commits_v1(f, index, index_nr, commit_positions);
 
 	if (options & BITMAP_OPT_HASH_CACHE)
 		write_hash_cache(f, index, index_nr);
@@ -730,4 +740,5 @@ void bitmap_writer_finish(struct pack_idx_entry **index,
 		die_errno("unable to rename temporary bitmap file to '%s'", filename);
 
 	strbuf_release(&tmp_file);
+	free(commit_positions);
 }