git-repack --max-pack-size: add option parsing to enable feature

Add --max-pack-size parsing and usage messages.
Upgrade git-repack.sh to handle multiple packfile names,
and build packfiles in GIT_OBJECT_DIRECTORY not GIT_DIR.
Update documentation.

Signed-off-by: Dana L. How <danahow@gmail.com>
Signed-off-by: Junio C Hamano <junkio@cox.net>
diff --git a/Documentation/git-pack-objects.txt b/Documentation/git-pack-objects.txt
index 2531238..cfe127a 100644
--- a/Documentation/git-pack-objects.txt
+++ b/Documentation/git-pack-objects.txt
@@ -85,6 +85,11 @@
 	times to get to the necessary object.
 	The default value for --window is 10 and --depth is 50.
 
+--max-pack-size=<n>::
+	Maximum size of each output packfile, expressed in MiB.
+	If specified,  multiple packfiles may be created.
+	The default is unlimited.
+
 --incremental::
 	This flag causes an object already in a pack ignored
 	even if it appears in the standard input.