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>

Makefile

@@ -209,7 +209,8 @@ SCRIPT_SH = \
 	git-am.sh \
 	git-merge.sh git-merge-stupid.sh git-merge-octopus.sh \
 	git-merge-resolve.sh git-merge-ours.sh \
-	git-lost-found.sh git-quiltimport.sh git-submodule.sh
+	git-lost-found.sh git-quiltimport.sh git-submodule.sh \
+	git-filter-branch.sh
 
 SCRIPT_PERL = \
 	git-add--interactive.perl \