midx-write.c: factor fanout layering from compute_sorted_entries()

When computing the set of objects to appear in a MIDX, we use
compute_sorted_entries(), which handles objects from various existing
sources one fanout layer at a time.

The process for computing this set is slightly different during MIDX
compaction, so factor out the existing functionality into its own
routine to prevent `compute_sorted_entries()` from becoming too
difficult to read.

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

Author: ttaylorr

midx-write.c

@@ -323,6 +323,30 @@ static void midx_fanout_add_pack_fanout(struct midx_fanout *fanout,
 	}
 }
 
+static void midx_fanout_add(struct midx_fanout *fanout,
+			    struct write_midx_context *ctx,
+			    uint32_t start_pack,
+			    uint32_t cur_fanout)
+{
+	uint32_t cur_pack;
+
+	if (ctx->m && !ctx->incremental)
+		midx_fanout_add_midx_fanout(fanout, ctx->m, cur_fanout,
+					    ctx->preferred_pack_idx);
+
+	for (cur_pack = start_pack; cur_pack < ctx->nr; cur_pack++) {
+		int preferred = cur_pack == ctx->preferred_pack_idx;
+		midx_fanout_add_pack_fanout(fanout, ctx->info, cur_pack,
+					    preferred, cur_fanout);
+	}
+
+	if (ctx->preferred_pack_idx != NO_PREFERRED_PACK &&
+	    ctx->preferred_pack_idx < start_pack)
+		midx_fanout_add_pack_fanout(fanout, ctx->info,
+					    ctx->preferred_pack_idx, 1,
+					    cur_fanout);
+}
+
 /*
  * It is possible to artificially get into a state where there are many
  * duplicate copies of objects. That can create high memory pressure if
@@ -359,23 +383,7 @@ static void compute_sorted_entries(struct write_midx_context *ctx,
 	for (cur_fanout = 0; cur_fanout < 256; cur_fanout++) {
 		fanout.nr = 0;
 
-		if (ctx->m && !ctx->incremental)
-			midx_fanout_add_midx_fanout(&fanout, ctx->m, cur_fanout,
-						    ctx->preferred_pack_idx);
-
-		for (cur_pack = start_pack; cur_pack < ctx->nr; cur_pack++) {
-			int preferred = cur_pack == ctx->preferred_pack_idx;
-			midx_fanout_add_pack_fanout(&fanout,
-						    ctx->info, cur_pack,
-						    preferred, cur_fanout);
-		}
-
-		if (ctx->preferred_pack_idx != NO_PREFERRED_PACK &&
-		    ctx->preferred_pack_idx < start_pack)
-			midx_fanout_add_pack_fanout(&fanout, ctx->info,
-						    ctx->preferred_pack_idx, 1,
-						    cur_fanout);
-
+		midx_fanout_add(&fanout, ctx, start_pack, cur_fanout);
 		midx_fanout_sort(&fanout);
 
 		/*