pack-objects: introduce pack.allowPackReuse

Let's make it possible to configure if we want pack reuse or not.

The main reason it might not be wanted is probably debugging and
performance testing, though pack reuse _might_ cause larger packs,
because we wouldn't consider the reused objects as bases for
finding new deltas.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/config/pack.txt

@@ -27,6 +27,13 @@ Note that changing the compression level will not automatically recompress
 all existing objects. You can force recompression by passing the -F option
 to linkgit:git-repack[1].
 
+pack.allowPackReuse::
+	When true, and when reachability bitmaps are enabled,
+	pack-objects will try to send parts of the bitmapped packfile
+	verbatim. This can reduce memory and CPU usage to serve fetches,
+	but might result in sending a slightly larger pack. Defaults to
+	true.
+
 pack.island::
 	An extended regular expression configuring a set of delta
 	islands. See "DELTA ISLANDS" in linkgit:git-pack-objects[1]

builtin/pack-objects.c

@@ -96,6 +96,7 @@ static off_t reuse_packfile_offset;
 
 static int use_bitmap_index_default = 1;
 static int use_bitmap_index = -1;
+static int allow_pack_reuse = 1;
 static enum {
 	WRITE_BITMAP_FALSE = 0,
 	WRITE_BITMAP_QUIET,
@@ -2715,6 +2716,10 @@ static int git_pack_config(const char *k, const char *v, void *cb)
 		use_bitmap_index_default = git_config_bool(k, v);
 		return 0;
 	}
+	if (!strcmp(k, "pack.allowpackreuse")) {
+		allow_pack_reuse = git_config_bool(k, v);
+		return 0;
+	}
 	if (!strcmp(k, "pack.usesparse")) {
 		sparse = git_config_bool(k, v);
 		return 0;
@@ -3050,7 +3055,8 @@ static void loosen_unused_packed_objects(void)
  */
 static int pack_options_allow_reuse(void)
 {
-	return pack_to_stdout &&
+	return allow_pack_reuse &&
+	       pack_to_stdout &&
 	       allow_ofs_delta &&
 	       !ignore_packed_keep_on_disk &&
 	       !ignore_packed_keep_in_core &&