Don't allow fast-import tree delta chains to exceed maximum depth

Brian Downing noticed fast-import can produce tree depths of up
to 6,035 objects and even deeper.  Long delta chains can create
very small packfiles but cause problems during repacking as git
needs to unpack each tree to count the reachable blobs.

What's happening here is the active branch cache isn't big enough.
We're swapping out the branch and thus recycling the tree information
(struct tree_content) back into the free pool.  When we later reload
the tree we set the delta_depth to 0 but we kept the tree we just
reloaded as a delta base.

So if the tree we reloaded was already at the maximum depth we
wouldn't know it and make the new tree a delta.  Multiply the
number of times the branch cache has to swap out the tree times
max_depth (10) and you get the maximum delta depth of a tree created
by fast-import.  In Brian's case above the active branch cache had
to swap the branch out 603/604 times during this import to produce
a tree with a delta depth of 6035.

Acked-by: Brian Downing <bdowning@lavos.net>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

fast-import.c

@@ -154,13 +154,16 @@ Format of STDIN stream:
 
 #define PACK_ID_BITS 16
 #define MAX_PACK_ID ((1<<PACK_ID_BITS)-1)
+#define DEPTH_BITS 13
+#define MAX_DEPTH ((1<<DEPTH_BITS)-1)
 
 struct object_entry
 {
 	struct object_entry *next;
 	uint32_t offset;
-	unsigned type : TYPE_BITS;
-	unsigned pack_id : PACK_ID_BITS;
+	uint32_t type : TYPE_BITS,
+		pack_id : PACK_ID_BITS,
+		depth : DEPTH_BITS;
 	unsigned char sha1[20];
 };
 
@@ -1105,7 +1108,7 @@ static int store_object(
 		unsigned pos = sizeof(hdr) - 1;
 
 		delta_count_by_type[type]++;
-		last->depth++;
+		e->depth = last->depth + 1;
 
 		hdrlen = encode_header(OBJ_OFS_DELTA, deltalen, hdr);
 		write_or_die(pack_data->pack_fd, hdr, hdrlen);
@@ -1117,8 +1120,7 @@ static int store_object(
 		write_or_die(pack_data->pack_fd, hdr + pos, sizeof(hdr) - pos);
 		pack_size += sizeof(hdr) - pos;
 	} else {
-		if (last)
-			last->depth = 0;
+		e->depth = 0;
 		hdrlen = encode_header(type, datlen, hdr);
 		write_or_die(pack_data->pack_fd, hdr, hdrlen);
 		pack_size += hdrlen;
@@ -1134,6 +1136,7 @@ static int store_object(
 			free(last->data);
 		last->data = dat;
 		last->offset = e->offset;
+		last->depth = e->depth;
 		last->len = datlen;
 	}
 	return 0;
@@ -1181,7 +1184,7 @@ static void load_tree(struct tree_entry *root)
 	if (myoe && myoe->pack_id != MAX_PACK_ID) {
 		if (myoe->type != OBJ_TREE)
 			die("Not a tree: %s", sha1_to_hex(sha1));
-		t->delta_depth = 0;
+		t->delta_depth = myoe->depth;
 		buf = gfi_unpack_entry(myoe, &size);
 	} else {
 		enum object_type type;
@@ -2347,8 +2350,11 @@ int main(int argc, const char **argv)
 		}
 		else if (!prefixcmp(a, "--max-pack-size="))
 			max_packsize = strtoumax(a + 16, NULL, 0) * 1024 * 1024;
-		else if (!prefixcmp(a, "--depth="))
+		else if (!prefixcmp(a, "--depth=")) {
 			max_depth = strtoul(a + 8, NULL, 0);
+			if (max_depth > MAX_DEPTH)
+				die("--depth cannot exceed %u", MAX_DEPTH);
+		}
 		else if (!prefixcmp(a, "--active-branches="))
 			max_active_branches = strtoul(a + 18, NULL, 0);
 		else if (!prefixcmp(a, "--import-marks="))