Documentation: filter-branch: document how to filter all refs

Document the '--' option that can be used to pass rev-list options
(not just arguments), and give an example usage of '-- --all'.  Remove
reference to "the new branch name"; filter-branch takes arbitrary
arguments to rev-list since dfd05e3.

Signed-off-by: Thomas Rast <trast@student.ethz.ch>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-filter-branch.txt

@@ -13,7 +13,7 @@ SYNOPSIS
 	[--msg-filter <command>] [--commit-filter <command>]
 	[--tag-name-filter <command>] [--subdirectory-filter <directory>]
 	[--original <namespace>] [-d <directory>] [-f | --force]
-	[<rev-list options>...]
+	[--] [<rev-list options>...]
 
 DESCRIPTION
 -----------
@@ -168,10 +168,10 @@ to other tags will be rewritten to point to the underlying commit.
 	'refs/original/', unless forced.
 
 <rev-list options>...::
-	When options are given after the new branch name, they will
-	be passed to 'git-rev-list'.  Only commits in the resulting
-	output will be filtered, although the filtered commits can still
-	reference parents which are outside of that set.
+	Arguments for 'git-rev-list'.  All positive refs included by
+	these options are rewritten.  You may also specify options
+	such as '--all', but you must use '--' to separate them from
+	the 'git-filter-branch' options.
 
 
 Examples
@@ -196,6 +196,17 @@ git filter-branch --index-filter 'git rm --cached filename' HEAD
 
 Now, you will get the rewritten history saved in HEAD.
 
+To rewrite the repository to look as if `foodir/` had been its project
+root, and discard all other history:
+
+-------------------------------------------------------
+git filter-branch --subdirectory-filter foodir -- --all
+-------------------------------------------------------
+
+Thus you can, e.g., turn a library subdirectory into a repository of
+its own.  Note the `\--` that separates 'filter-branch' options from
+revision options, and the `\--all` to rewrite all branches and tags.
+
 To set a commit (which typically is at the tip of another
 history) to be the parent of the current initial commit, in
 order to paste the other history behind the current history: