don't let the delta cache grow unbounded in 'git repack'

I have 4GB of RAM on my system which should, in theory, be quite enough
to repack a 600 MB repository.  However the unbounded delta cache size
always pushes it into swap, at which point everything virtually comes to
a halt.  So unbounded caches are never a good idea.

A default of 256MB should be a good compromize between memory usage and
speed where medium sized repositories are still likely to fit in the
cache with a reasonable memory usage, and larger repositories are going
to take quite some time to repack already anyway.

While at it, clarify the associated config variable documentation
entries a bit.

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

Documentation/config.txt

@@ -1218,12 +1218,20 @@ pack.compression::
 
 pack.deltaCacheSize::
 	The maximum memory in bytes used for caching deltas in
-	linkgit:git-pack-objects[1].
-	A value of 0 means no limit. Defaults to 0.
+	linkgit:git-pack-objects[1] before writing them out to a pack.
+	This cache is used to speed up the writing object phase by not
+	having to recompute the final delta result once the best match
+	for all objects is found.  Repacking large repositories on machines
+	which are tight with memory might be badly impacted by this though,
+	especially if this cache pushes the system into swapping.
+	A value of 0 means no limit. The smallest size of 1 byte may be
+	used to virtually disable this cache. Defaults to 256 MiB.
 
 pack.deltaCacheLimit::
 	The maximum size of a delta, that is cached in
-	linkgit:git-pack-objects[1]. Defaults to 1000.
+	linkgit:git-pack-objects[1]. This cache is used to speed up the
+	writing object phase by not having to recompute the final delta
+	result once the best match for all objects is found. Defaults to 1000.
 
 pack.threads::
 	Specifies the number of threads to spawn when searching for best

builtin-pack-objects.c

@@ -86,7 +86,7 @@ static int pack_compression_level = Z_DEFAULT_COMPRESSION;
 static int pack_compression_seen;
 
 static unsigned long delta_cache_size = 0;
-static unsigned long max_delta_cache_size = 0;
+static unsigned long max_delta_cache_size = 256 * 1024 * 1024;
 static unsigned long cache_max_small_delta_size = 1000;
 
 static unsigned long window_memory_limit = 0;