Add git-filter-branch

This script is derived from Pasky's cg-admin-rewritehist.

In fact, it _is_ the same script, minimally adapted to work without cogito.
It _should_ be able to perform the same tasks, even if only relying on
core-git programs.

All the work is Pasky's, just the adaption is mine.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Hopefully-signed-off-by: Petr "cogito master" Baudis <pasky@suse.cz>
Signed-off-by: Junio C Hamano <junkio@cox.net>

t/t7003-filter-branch.sh

@@ -0,0 +1,47 @@
+#!/bin/sh
+
+test_description='git-filter-branch'
+. ./test-lib.sh
+
+make_commit () {
+	lower=$(echo $1 | tr A-Z a-z)
+	echo $lower > $lower
+	git add $lower
+	git commit -m $1
+	git tag $1
+}
+
+test_expect_success 'setup' '
+	make_commit A
+	make_commit B
+	git checkout -b branch B
+	make_commit D
+	make_commit E
+	git checkout master
+	make_commit C
+	git checkout branch
+	git merge C
+	git tag F
+	make_commit G
+	make_commit H
+'
+
+H=$(git-rev-parse H)
+
+test_expect_success 'rewrite identically' '
+	git-filter-branch H2
+'
+
+test_expect_success 'result is really identical' '
+	test $H = $(git-rev-parse H2)
+'
+
+test_expect_success 'rewrite, renaming a specific file' '
+	git-filter-branch --tree-filter "mv d doh || :" H3
+'
+
+test_expect_success 'test that the file was renamed' '
+	test d = $(git show H3:doh)
+'
+
+test_done