Document git-filter-branch

This moves the documentation in git-filter-branch.sh to its own
man page, with a few touch ups (incorporating comments by Frank
Lichtenheld).

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/cmd-list.perl

@@ -105,6 +105,7 @@ git-diff-tree                           plumbinginterrogators
 git-fast-import				ancillarymanipulators
 git-fetch                               mainporcelain
 git-fetch-pack                          synchingrepositories
+git-filter-branch                       ancillarymanipulators
 git-fmt-merge-msg                       purehelpers
 git-for-each-ref                        plumbinginterrogators
 git-format-patch                        mainporcelain