threaded delta search: add pack.threads config variable

Signed-off-by: NNicolas Pitre <nico@cam.org>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

Documentation/config.txt

@@ -630,9 +630,17 @@ pack.deltaCacheSize::
 	A value of 0 means no limit. Defaults to 0.
 
 pack.deltaCacheLimit::
-	The maxium size of a delta, that is cached in
+	The maximum size of a delta, that is cached in
 	gitlink:git-pack-objects[1]. Defaults to 1000.
 
+pack.threads::
+	Specifies the number of threads to spawn when searching for best
+	delta matches.  This requires that gitlink:git-pack-objects[1]
+	be compiled with pthreads otherwise this option is ignored with a
+	warning. This is meant to reduce packing time on multiprocessor
+	machines. The required amount of memory for the delta search window
+	is however multiplied by the number of threads.
+
 pull.octopus::
 	The default merge strategy to use when pulling multiple branches
 	at once.

builtin-pack-objects.c

@@ -1746,6 +1746,17 @@ static int git_pack_config(const char *k, const char *v)
 		cache_max_small_delta_size = git_config_int(k, v);
 		return 0;
 	}
+	if (!strcmp(k, "pack.threads")) {
+		delta_search_threads = git_config_int(k, v);
+		if (delta_search_threads < 1)
+			die("invalid number of threads specified (%d)",
+			    delta_search_threads);
+#ifndef THREADED_DELTA_SEARCH
+		if (delta_search_threads > 1)
+			warning("no threads support, ignoring %s", k);
+#endif
+		return 0;
+	}
 	return git_default_config(k, v);
 }