git-filter-branch: document --original option

Signed-off-by: Giuseppe Bilotta <giuseppe.bilotta@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-filter-branch.txt

@@ -12,7 +12,8 @@ SYNOPSIS
 	[--index-filter <command>] [--parent-filter <command>]
 	[--msg-filter <command>] [--commit-filter <command>]
 	[--tag-name-filter <command>] [--subdirectory-filter <directory>]
-	[-d <directory>] [-f | --force] [<rev-list options>...]
+	[--original <namespace>] [-d <directory>] [-f | --force]
+	[<rev-list options>...]
 
 DESCRIPTION
 -----------
@@ -134,6 +135,10 @@ definition impossible to preserve signatures at any rate.)
 	The result will contain that directory (and only that) as its
 	project root.
 
+--original <namespace>::
+	Use this option to set the namespace where the original commits
+	will be stored. The default value is 'refs/original'.
+
 -d <directory>::
 	Use this option to set the path to the temporary directory used for
 	rewriting.  When applying a tree filter, the command needs to