pack-objects: document --delta-base-offset option

Signed-off-by: Junio C Hamano <junkio@cox.net>

Documentation/git-pack-objects.txt

@@ -9,7 +9,7 @@ git-pack-objects - Create a packed archive of objects
 SYNOPSIS
 --------
 [verse]
-'git-pack-objects' [-q] [--no-reuse-delta] [--non-empty]
+'git-pack-objects' [-q] [--no-reuse-delta] [--delta-base-offset] [--non-empty]
 	[--local] [--incremental] [--window=N] [--depth=N]
 	[--revs [--unpacked | --all]*] [--stdout | base-name] < object-list
 
@@ -110,6 +110,17 @@ base-name::
 	This flag tells the command not to reuse existing deltas
 	but compute them from scratch.
 
+--delta-base-offset::
+	A packed archive can express base object of a delta as
+	either 20-byte object name or as an offset in the
+	stream, but older version of git does not understand the
+	latter.  By default, git-pack-objects only uses the
+	former format for better compatibility.  This option
+	allows the command to use the latter format for
+	compactness.  Depending on the average delta chain
+	length, this option typically shrinks the resulting
+	packfile by 3-5 per-cent.
+
 
 Author
 ------

builtin-pack-objects.c

@@ -15,7 +15,7 @@
 #include <sys/time.h>
 #include <signal.h>
 
-static const char pack_usage[] = "git-pack-objects [-q] [--no-reuse-delta] [--non-empty] [--local] [--incremental] [--window=N] [--depth=N] [--revs [--unpacked | --all]*] [--stdout | base-name] <ref-list | <object-list]";
+static const char pack_usage[] = "git-pack-objects [-q] [--no-reuse-delta] [--delta-base-offset] [--non-empty] [--local] [--incremental] [--window=N] [--depth=N] [--revs [--unpacked | --all]*] [--stdout | base-name] <ref-list | <object-list]";
 
 struct object_entry {
 	unsigned char sha1[20];