pack-objects: use bitfield for object_entry::depth

Because of struct packing from now on we can only handle max depth
4095 (or even lower when new booleans are added in this struct). This
should be ok since long delta chain will cause significant slow down
anyway.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/config.txt

@@ -2422,6 +2422,7 @@ pack.window::
 pack.depth::
 	The maximum delta depth used by linkgit:git-pack-objects[1] when no
 	maximum depth is given on the command line. Defaults to 50.
+	Maximum value is 4095.
 
 pack.windowMemory::
 	The maximum size of memory that is consumed by each thread

Documentation/git-pack-objects.txt

@@ -96,7 +96,9 @@ base-name::
 	it too deep affects the performance on the unpacker
 	side, because delta data needs to be applied that many
 	times to get to the necessary object.
-	The default value for --window is 10 and --depth is 50.
++
+The default value for --window is 10 and --depth is 50. The maximum
+depth is 4095.
 
 --window-memory=<n>::
 	This option provides an additional limit on top of `--window`;

Documentation/git-repack.txt

@@ -90,7 +90,9 @@ other objects in that pack they already have locally.
 	space. `--depth` limits the maximum delta depth; making it too deep
 	affects the performance on the unpacker side, because delta data needs
 	to be applied that many times to get to the necessary object.
-	The default value for --window is 10 and --depth is 50.
++
+The default value for --window is 10 and --depth is 50. The maximum
+depth is 4095.
 
 --threads=<n>::
 	This option is passed through to `git pack-objects`.

builtin/pack-objects.c

@@ -3068,6 +3068,12 @@ int cmd_pack_objects(int argc, const char **argv, const char *prefix)
 	if (pack_to_stdout != !base_name || argc)
 		usage_with_options(pack_usage, pack_objects_options);
 
+	if (depth >= (1 << OE_DEPTH_BITS)) {
+		warning(_("delta chain depth %d is too deep, forcing %d"),
+			depth, (1 << OE_DEPTH_BITS) - 1);
+		depth = (1 << OE_DEPTH_BITS) - 1;
+	}
+
 	argv_array_push(&rp, "pack-objects");
 	if (thin) {
 		use_internal_rev_list = 1;

pack-objects.h

@@ -2,6 +2,7 @@
 #define PACK_OBJECTS_H
 
 #define OE_DFS_STATE_BITS	2
+#define OE_DEPTH_BITS		12
 
 /*
  * State flags for depth-first search used for analyzing delta cycles.
@@ -89,9 +90,7 @@ struct object_entry {
 	unsigned tagged:1; /* near the very tip of refs */
 	unsigned filled:1; /* assigned write-order */
 	unsigned dfs_state:OE_DFS_STATE_BITS;
-
-	int depth;
-
+	unsigned depth:OE_DEPTH_BITS;
 };
 
 struct packing_data {